Proximity detection

ABSTRACT

An N-M tomography system comprising: a carrier for the subject of an examination procedure; a plurality of detector heads; a carrier for the detector heads; and a detector positioning arrangement operable to position the detector heads during performance of a scan without interference or collision between adjacent detector heads to establish a variable bore size and configuration for the examination. Additionally, collimated detectors providing variable spatial resolution for SPECT imaging and which can also be used for PET imaging, whereby one set of detectors can be selectably used for either modality, or for both simultaneously.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/399,975 filed Nov. 10, 2014, which is a National Phase of PCT PatentApplication No. PCT/IB2013/053721 having International Filing Date ofMay 8, 2013, which is a continuation in part of, and claims the benefitof priority under 35 USC 119(e) of U.S. Provisional Patent ApplicationNo. 61/644,120 filed on May 8, 2012, U.S. Provisional Patent ApplicationNo. 61/646,333 filed on May 13, 2012, and U.S. Provisional PatentApplication No. 61/788,394 filed on Mar. 15, 2013. The contents of theabove applications are all incorporated by reference as if fully setforth in their entirety.

FIELD OF THE INVENTION

The present invention, in some of its embodiments, relates to apparatusand methods of tomography in the field of nuclear medicine (sometimesreferred to herein as “N-M tomography”), and more particularly to N-Mtomography systems as a whole, collimated detector configurations forN-M tomography systems, and methods of using N-M tomography systems anddetectors. In some embodiments, the invention relates to modular N-Mtomography systems and to methods of upgrading existing N-M tomographysystems. In some embodiments, the invention relates to dual useSPECT-PET N-M tomography systems and detectors.

BACKGROUND OF THE INVENTION In General:

Systems and methods for medical imaging based on detection of particlesemitted by decay of radioactive tracer compounds injected into thebloodstream of a subject have become important diagnostic and researchtools, for example, in the fields of cardiology, oncology, andneurology, as well in pre-clinical (e.g., small-animal) studies.

There are two major types of such imaging systems, single photonemission tomography (SPECT) and positron emission tomography (PET).SPECT and PET systems both rely on detection of gamma-ray photonsresulting from decay of radio-isotopes, the concentration of which intissues is indicative of metabolic or other processes within thetissues. For example, tumors are characterized by high metabolicactivity and therefore high uptake of the tracer compound, andcorrespondingly, higher gamma photon emission, while normal tissueexhibits relatively lower metabolic activity and therefore lower uptakeof the tracer compound and lower photon emission.

Emission data is obtained at multiple positions around and along theregion of interest (ROI) to produce data representing a succession of 2Dimages or projections which are then converted by use of suitablecomputerized image reconstruction algorithms into 3D images.

FIG. 1A illustrates schematically major external features of anexemplary N-M tomography system, generally designated at 10. Theillustration is broadly representative of the major external features ofboth conventional SPECT and PET systems, and of some embodiments of thepresent invention.

System 10 is comprised of a gantry 12 mounted in a stationary frame 14,and a patient carrier 16 including a patient bed arranged to movevertically within the gantry, and axially through the gantry on a track19. In some installations, patient carrier 16 is stationary, and gantry12 is axially moveable along the patient carrier.

In conventional SPECT systems, gantry 12 is a ring-like structurearranged to rotate in frame 14 around an axis that runs along the lengthof the patient carrier 16. (For convenience, this will be referred toherein as the “system axis” or the “patient axis”.) The emissiondetectors in SPECT systems, or “cameras” as they are usually referredto, are typically comprised of a small number, for example, one to fourdetector units configured as flat plates, sometimes square and sometimesrectangular, extending longitudinally in the direction of the systemaxis, i.e., into the plane of FIGS. 1A-1B. When two detector units areused, they are often positioned 90 degrees apart on the gantry.

Relative axial movement between the gantry and the patient bed producesthe succession of 2D emission projections from which the 3D images arereconstructed. After reconstruction, a “slice” corresponding to an axialposition may be selected.

FIG. 1B shows another SPECT system configuration that is particularlyuseful for cardiac studies of various kinds. The device, generallydenoted at 20 employs an adjustable chair 22 as the patient carrier. Thedetectors are located in a housing 24 moveably attached to a main systemhousing 26.

The detector housing 24 is generally L-shaped, and the detectors shownby dash lines 28 extend transversely to the housing and generally alongthe patient axis in an arc around the patient's torso.

A SPECT system configured as illustrated is available commercially atthe time of filing of this application from the applicant hereof underthe name D-Spect®.

As noted above, the external features of conventional PET systems aresimilar to those of conventional SPECT systems as illustrated in FIG.1A. However, the detector heads of a PET system are typically mounted ona non-rotating gantry ring surrounding the patient bed. In typicalcommercial installations, the gantry ring is fully populated by a largenumber of closely spaced detectors, each comprised of one or moreindividual detector elements having small areas, to provide 360 degreecoverage around an ROI. PET detectors typically do not includecollimators.

Other configurations have also been proposed. U.S. Pat. No. 6,137,109pertains to SPECT and positron coincidence detection (PET) systemshaving a small number of detector heads, e.g., two or three, arranged ina polygonal configuration around a gantry. Provision is made for thedetector heads to be moved radially toward the patient examination areaand tangentially on the gantry to preset the bore size, before anexamination is performed.

Cherry, et al., Physics in Nuclear Medicine, Third Ed. p. 346, shows aPET system having a gantry ring partially populated by a small number ofdetector heads arranged in a uniformly or non-uniformly spacedrelationship on the ring. In such constructions, to obtain data neededfor full image reconstruction, i.e., to provide full 360 degreecoverage, the gantry must be rotated to a succession of angularpositions. However, a typical gantry installation including detectorhead positioning mechanisms can weigh hundreds of kilograms, andrepositioning the gantry between images can require several minutes.This can be a significant factor in the time required for a scan.Possibly due to the lower efficiency and prolonged acquisition time,fixed-diameter rotating-only partly populated gantries have not beenadopted commercially.

A PET installation also generally includes a cyclotron (not shown) whichis needed for production of the isotopes used as tracers on site, or ata nearby facility. These isotopes have very short half-lives, i.e. theydecay quite rapidly, and often cannot practically be used if producedoff-site at a distant facility.

Some Differences Between PET and SPECT Systems and Methods:

SPECT and PET systems are structurally and functionally different inseveral important respects, and have generally different advantages andlimitations.

Basic Physics:

In PET imaging, as a radioactive isotope of the tracer undergoes decay,it emits a positron, an antiparticle of the electron with oppositecharge. The emitted positron travels in tissue for a short distance(typically about 1 mm), losing energy as it travels, to a point where itcan interact with an electron. The interaction annihilates both theelectron and the positron, producing a pair of gamma ray photons havingenergies of (typically) 511 KeV that move in approximately oppositedirections. These high energy photons are detected as a “paired event”(often referred to as “coincidence”) by an opposed pair of detectors.Signals from the detectors are collected and temporally correlated tofind such pairs and used to generate or reconstruct the 3D images.

For SPECT imaging, radioactive decay of the tracer isotope results inemission of single gamma-ray photons without the intermediate step ofpositron-electron annihilation. These photons typically have energies inthe range of about 40-245 KeV, and are detected as singular events.

Radiotracers:

Isotopes used in PET imaging typically have short half-lives (i.e.,exhibit rapid decay). Examples include carbon-11 (approximately 20 minhalf-life), nitrogen-13 (approximately 10 min), oxygen-15 (approximately2 min), fluorine-18 (approximately 110 min), or rubidum-82(approximately 1.27 min) The most commonly used radiotracer in PETimaging is fluorodeoxyglucose (also called FDG or fludeoxyglucose), ananalogue of glucose labeled with fluorine-18.

For SPECT imaging, the radioisotopes can be a simple soluble dissolvedion, such as a radioisotope of gallium. Alternatively, and mostcommonly, a marker radioisotope is attached to another compound which isof interest for its ability to bind in a medically interesting way totissue under investigation.

Typical tracers used in SPECT imaging include technetium-99m1 (the mostcommonly used), iodine-123, or iodine 131, or indium-111. These isotopesare heavier than those used in PET and exhibit half-lives measured inhours or even days.

As noted above, cells exhibiting high metabolic activity, for example,cancer cells, typically strongly absorb the radioisotope. In both PETand SPECT imaging, the intensity of the gamma-ray emission provides ameasure of concentration in the tissue.

Detectors:

Since both PET and SPECT imaging involve detection of gamma photons,both can utilize detectors that function by the same basic operatingprinciple. The original SPECT detector was the so-called “Anger Camera”(named after its inventor) developed in the 1950's, and while thetechnology has evolved over time, detectors employing the operatingprinciple of the Anger camera are still used in the majority ofcommercial SPECT installations.

FIG. 2A shows schematically the construction of an Anger Camera asconfigured for SPECT imaging, generally designated at 30. Camera 30 iscomprised of a detector element 32, usually formed of a thallium-dopedsodium iodide (NaI:Tl) crystal optically coupled to an array ofphotomultipliers (PMT) 34 positioned behind detector element 32.Typically, there are 30-60 or more PMTs depending on the sizes of thedetector and the PMTs, arranged in a square or rectangular array.

NaI:Tl crystal 32 functions as a scintillator to convert gamma-rayemissions from a radioactive tracer that has been injected into theblood stream of a subject 36 to visible or near ultraviolet lightpulses. PMTs 34 convert the light pulses produced by crystal 32 intoelectrical output signals which are provided to a computer 38 programmedaccording to a desired algorithm to reconstruct a succession of 2D sliceimages into an ultimate 3D image. The 3D image is then storedelectronically and/or provided to a visual display unit 40.

Many or all of PMTs 34 simultaneously detect the (presumed) same flashof light to varying degrees, at intensities generally depending on theirposition relative to the actual emission event. In conventionalpractice, spatial information about the locations of the gamma photonemissions is obtained by placing a collimator 42 in front of thedetection crystal/PMT array. The collimator consists of a thick sheet oflead, typically 1-3 inches thick, with thousands of adjacent holesthrough it which limits the direction from which the photons can reachcrystal 32. The holes are sometimes defined by septa, which blockradiation at other directions. In some cases, the collimator is amachined metal plate.

SPECT cameras and especially Anger-type SPECT cameras, use collimatorsto ensure that photons striking the detectors do so at a relativelynarrow range of angles. However, the collimator absorbs a substantialpercentage of the incident photons. This limits the sensitivity of thecamera system, thereby increasing the time required to obtain sufficientdata for a good image. Other methods of image localization, for example,different collimator configurations, and other scintillation detectormaterials have been proposed but the classic Anger camera stilldominates in commercial use. Compton cameras do not use a collimator fordetecting the direction, instead scattering of the gamma ray from oneset of detectors is detected by a second set of detectors and the pathof the gamma ray reconstructed therefrom.

Also known are direct conversion semiconductors that respond directly togamma-ray photons to produce electrical output signals, thus eliminatingthe need for the PMTs. Silicon-based devices, sometimes referred to assilicon photomultipliers (or SiPMs) are described in Roncali et al.,Application of Silicon Photomultipliers to Positron Emission Tomography,Ann Biomed Eng. 2011 April; 39(4): 1358-1377, the content of which ishereby incorporated in its entirety herein. One example of directconversion detectors are detectors formed of cadmium zinc telluride(CZT) or other materials.

FIG. 2B is a block diagram that illustrates schematically majorelectronic features of an exemplary conventional PET system, generallydesignated at 50. Illustrative system 50 includes a detector signalinput unit 52 which receives and pre-processes the signal outputs of thedetector heads received over a set of signal paths 54, As will beunderstood, for PET scans, a main function of input unit 54 is toperform coincidence detection of paired signals. As will also beunderstood, for SPECT imaging, the detector head outputs are processedwithout the need for coincidence detection.

Output signals from Input Unit 22 are provided to an ImageReconstruction Unit 56, typically a suitably programmedmicroprocessor-based system, by which the 3D images of the ROIs arecreated from the detected photon patterns.

The reconstructed images are provided to a display sub-system 58,typically including a user interface, e.g., a keyboard and mouse orother input device (not shown) and a display unit 58.

The operation of the system of FIG. 2B is controlled by a systemcontroller 30, which is typically comprised of a suitably programmedmicroprocessor, possibly the same one that performs imagereconstruction.

As mentioned above, PET imaging relies on coincidence detection of twooppositely traveling photons, and therefore different detectorconfigurations are conventionally used for PET and SPECT imaging. Onenotable difference is that collimators are not needed for PET imagingsince the required spatial resolution results from the coincidencedetection process. The absence of a collimator also results in improvedsensitivity.

Correspondingly, different image reconstruction algorithms are used forPET and SPECT imaging. One PET image reconstruction algorithm known tothose skilled in the art include ML-EM algorithm. Among SPECT imagereconstruction algorithms known to those skilled in the art areiterative reconstruction and back projection.

Possibly relevant discussions of PET detector technology may be found inLewellen, Recent Developments in PET Detector Technology, Physics inMedicine and Biology, 53(2008) R-287-317, 11 Aug. 2008. Otherdiscussions possibly relevant to PET technology may be found in Cherry,et al., supra.

A comprehensive survey of SPECT detector technology may be found inPetersen and Furenlid, SPECT detectors: the Anger Camera and Beyond,Physics in Medicine and Biology, 56(2011) R-145-182, 9 Aug. 2011 and inCherry, et al., supra.

The contents of each of these documents are incorporated herein byreference as if fully set forth.

Both conventional PET and SPECT systems are widely used in the fields ofoncology and neurology, while SPECT systems are commonly used incardiology, bone scan imagining, and pre-clinical studies. Anotherfactor relevant to choice of one or the other modality is availabilityof the isotopes and tracers. Tc-99m is very commonly available for SPECTwhile F-18 is commonly available for PET.

Each modality is commonly accepted to have certain advantages andlimitations. For example, PET systems have historically provided betterspatial resolution and faster examination, but are generally much moreexpensive than SPECT systems, mainly due to the large number of detectorelements needed to fully populate the gantry, and the complexcoincidence circuitry, but also partially due to the fact that thecyclotron itself is costly and requires high-level technical support.

SPECT systems yield good results for many general purpose applications,including in the fields of cardiology and small-animal pre-clinicalimaging. For example, because the gantry rotates during a scan,obtaining full 180 degree coverage may be easier. However, despitetechnological advances, conventional SPECT systems still provide lowerresolution images than PET systems, and consequently, the SPECT imagesare generally less detailed than PET images due at least in part toscatter and attenuation of the lower energy of the photons. A SPECT scanalso typically takes longer than a PET scan, and requires a patient notto move for a longer time, or even to be immobilized and limits theability to visualize rapid functional changes. Also, the PET tracers'high energy of the photons results in spatial resolution that is lessdependent on distance to the ROI.

Further, since SPECT detectors generally require collimators, there isan inherent tradeoff between resolution and sensitivity.

Another limitation of conventional N-M tomography systems, especiallyPET systems, is that reducing the number of detector heads to avoid thehigh cost of fully populating the gantry ring as in U.S. Pat. No.6,137,109 and in Cherry et al. mentioned above sometimes results inreduced spatial resolution and/or longer scan times for scans of smallROIs in systems having a bore size large enough to accommodate obesepatients. This limitation is often addressed by the purchase of separatesystems for examining patients of large girth and for examining smallROIs.

Yet a further issue concerns the high initial cost of an N-M tomographysystem. Often, a customer has a limited budget or limited needs whichresults in purchase of a system having limited capability. For example,a facility faced with budgetary limitations might choose to investinitially in a SPECT or a PET system, but not both, depending on patientvolumes and a decision to focus on cardiology or oncology and/orneurology. However, as financial resources or needs change, an entirelynew system may need to be purchased. The same thing may happen whentechnical advances dictate a need for improved capabilities.

Commonly owned technology that may be generally relevant to SPECTimaging, protocols and doses, reconstruction and multi-dimensionalimaging and image processing and is optionally used together with someembodiments of the invention may be found in one or more of thefollowing International (PCT) or U.S. published applications or issuedU.S. patents:

PCT Published Applications: WO/2006/129301; WO/2007/010537;WO/2007/010534; WO/2010/004536, WO/2007/010537; WO/2007/010534;WO/2007/010537; WO/2008/010227; WO/2010/004536 Published U.S.Applications:

2004/0015075; 2004/0054248; 2004/0204646; 2007/0194241; 2009/0112086;2008/0042067; 2008-0230705; 2008-0128626; 2008-0230702; 2009-0201291;2009-0304582; 2010-0021378; 2010-0142774; 2011-0026685; 2011-0112856;2012-0106820; 2012-0172699

U.S. Patents:

U.S. Pat. Nos. 6,173,201; 6,368,331; 6,567,687 7,652,259; 7,705,316;7,705,316; 7,826,889; 7,872,235; 7,968,851; 7,970,455; 8,000,773;8,036,731; 8,094,894; 8,111,886; 8,280,124; 8,338,788

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the invention, there isprovided an N-M tomography system having a support for a subject of anexamination procedure, a plurality of detector heads, a carrier for thedetector heads, and a detector positioning arrangement operable toposition the detector heads including rotation thereof to adjacent orradially or circumferentially or axially overlapping positions beforeand/or during performance of a scan without interference or collisionbetween adjacent detector heads to establish a variable bore size andconfiguration for the examination.

According to some embodiments, the detector heads include detectorswhich do not move relative to said heads during acquisition.

According to some embodiments, the system includes a processor whichuses data collected by all of said detector heads, for imagereconstruction.

According to some embodiments, the detector positioning arrangement isoperable to move at least some of the detectors toward and away from thesubject carrier to determine the detector bore size.

According to some embodiments, the movement of at least two detectorheads is off-center of a central axis of said bore and not towards anycommon parallel axis of said bore.

According to some embodiments, the movement of at least one detectorheads is not along a straight line.

According to some embodiments, the detector positioning arrangement isoperable to move at least some of the detectors to a desired rotationalorientation relative to respective longitudinal axes thereof.

According to some embodiments, the detector positioning arrangement isoperable to tilt at least some of the detector heads to a desiredorientation relative to respective longitudinal axes thereof.

According to some embodiments, the detector positioning arrangement isoperable to adjust the bore size preliminary to a scan, or in steps atselected axial and/or circumferential positions during a scan, orcontinuously as the scan proceeds.

According to some embodiments, the detector positioning arrangement iscomprised of separate mechanisms to extend and retract and to angularlyorient the detector heads.

According to some embodiments, detector-bearing parts of at least somedetector heads are tilted by rotation around an axis parallel to asystem axis.

According to some embodiments, one of the separate mechanisms forextending and retracting, and angularly orienting the detector heads isassociated with each detector head.

According to some embodiments, the detectors heads are all PET detectorheads or all SPECT detector heads, or a combination of SPECT and PETdetector heads or dual purpose detector heads.

According to some embodiments, the system includes a drive mechanism torotate the detector heads around the subject carrier to provide 360degree coverage around the patient carrier with substantially no gapsfor a selected bore size.

According to some embodiments, the detector heads are positionednon-uniformly around the detector carrier.

According to some embodiments, the detector positioning arrangement isoperable to orient the detector heads angularly to prevent interferencebetween adjacent detector heads when the detector heads are extended toa desired position.

According to some embodiments, the detector carrier is comprised of twoor more rings spaced apart from each other by an air gap along alongitudinal axis of the subject carrier with detector heads mounted oneach ring.

According to some embodiments, the detector carrier is a single ringwith detector heads axially spaced on one side of the single ring, or onopposite sides of the single ring.

According to some embodiments, a plurality of pairs of PET detectorheads are spaced circumferentially around the detector head carrier,such that adjacent pairs of PET detector heads are in a spacedrelationship on the longitudinal axis of the subject carrier.

According to some embodiments, at least some of the detector heads arecircumferentially or laterally moveable along a path near the peripheryof the detector carrier.

According to some embodiments, the detector carrier is configured toaccommodate a variable number of detector heads.

According to some embodiments, the system includes a controller whichcontrols the detector positioning arrangement to provide movements ofthe detectors.

According to some embodiments, each detector head is associated with itsown detector positioning arrangement.

According to an aspect of some embodiments of the invention, there isprovided an N-M tomography system having a plurality of detector headsmounted on a detector carrier that is configured to be populated by avariable number of detector heads and a data processing unit responsiveto signal outputs from the population of detector heads to reconstruct atomographic image from the signals.

According to some embodiments, the detector carrier of an existingsystem is configured to receive additional detector heads or replacementdetector heads having different operational characteristics as part of asystem upgrade.

According to some embodiments, the detector carrier includes mountingreceptacles to which the additional or replacement detector heads can beoperatively attached.

According to some embodiments, the detector heads include signalingmechanisms that indicate the presence of a detector head installed at aparticular position on the carrier, and/or the functionalcharacteristics of the installed detector heads.

According to some embodiments, the data processing unit is responsive tothe signaling mechanisms.

According to an aspect of some embodiments of the invention, there isprovided an N-M tomography system having a plurality of detector headsmounted on a detector carrier, wherein the detector carrier isconfigured to be populated by detector heads of variable type and/orquality; and a data processing unit responsive to signal outputs fromthe population of detector heads to reconstruct a tomographic image fromthe signals.

According to an aspect of some embodiments of the invention, there isprovided a method of using an N-M tomography system that includes asupport for a subject of a tomography procedure and a detectorarrangement comprised of plurality of detector heads that are adjustablypositionable on a detector carrier, in which the method involvesselecting a bore geometry for the detector array according to aparticular region of interest of a subject of a procedure by extendingand angularly orienting the detector heads such that adjacent detectorheads do not interfere with each other and, scanning the region ofinterest at successive axial positions relative to a patient carrier.

According to some embodiments, the bore size is selected by one or moreof: extending at least some of the detectors toward the subject carrier,rotating at least some of the detectors relative to respectivelongitudinal axes thereof, tilting at least some of the detectors to adesired off-axis orientation relative to respective longitudinal axesthereof, and moving at least some of the detector headscircumferentially or laterally relative to the periphery of the detectorcarrier.

According to some embodiments, the selected bore geometry is varied as ascan proceeds.

According to some embodiments, the bore geometry is varied for at leastsome axial positions.

According to some embodiments, the bore geometry is varied continuouslyas the scan proceeds at a particular axial position.

According to some embodiments, the detector carrier rotates as the scanproceeds at a particular axial position.

According to an aspect of some embodiments of the invention, there isprovided a method of upgrading an existing N-M tomography system thatincludes a support for a subject of a tomography procedure, a detectorarrangement comprised of a plurality of SPECT detector heads and/or PETdetector heads, in which one or more additional detector heads of adesired type are installed in receptacles at pre-existing couplingpositions on the detector carrier.

According to some embodiments, the method further includes installingadjacent PET detector pairs in receptacles at different axial spacingrelative to a subject carrier.

According to some embodiments, the method further includes positioningat least one additional detector carrier in spaced relationship with anexisting detector carrier along a longitudinal axis of the subjectcarrier.

According to some embodiments, an existing detector head is replaced inthe field, with a detector head of different quality or type.

According to an aspect of some embodiments of the invention, there isprovided a detector unit for a Nuclear Medicine (NM) imaging systemhaving a detector element responsive to gamma ray photons to provide anelectrical output signal and a collimator formed of first and secondsets of septa extending in two directions that intersect to define anarray of collimator cells, in which the septa are moveable to change thegeometry of the collimator to increase and decrease the spatialresolution of the detector unit.

According to some embodiments, the septa of the first set are equallyspaced from each other and the septa of the second set are equallyspaced from each other.

According to some embodiments, the spacing between the septa of thefirst set is equal to the spacing between the septa of the second set.

According to some embodiments, the spacing between the septa of thefirst set is unequal to the spacing between the septa of the second set.

According to some embodiments, all the septa of the first and secondsets are respectively parallel to each other.

According to some embodiments, all the septa of at least one of the setsare not parallel to each other.

According to some embodiments, the septa of at least one of the sets aremoveable to increase the length thereof relative to a surface of thedetector element.

According to some embodiments, the septa are moveable to increase ordecrease the septa spacing in one and/or both directions relative to asurface of the detector module.

According to some embodiments, the collimator is formed of a pluralityof parts spaced from each other in a direction perpendicular to asurface of the detector element, each part being formed of first andsecond sets of intersecting septa, and the spacing is increased ordecreased by moving one or both parts parallel to a surface of thedetector element.

According to some embodiments, the collimator is formed of two or threespaced parts.

According to some embodiments, the collimator is formed of three partsperpendicularly spaced relative to the detector element, wherein thesepta of the intermediate part are tiltable.

According to some embodiments, at least some the septa in one or bothsets are tiltable relative to a surface of the detector element.

According to some embodiments, one end of the collimator cells includesa shutter to adjust the effective size of the area exposed to incomingphotons.

According to some embodiments, the shutter is slidable or tiltable or inthe form of an iris.

According to some embodiments, the septa are comprised of plates havingspaced slots therein, wherein the spacing of the slots in the septa ofthe first set match the spacing of the septa of the second set, and thespacing of the slots in the septa of the second set match the spacing ofthe septa of the first set.

According to some embodiments, the septa of the first set are orientedorthogonally to the septa of the second set.

According to some embodiments, the spacing of the septa of the first andsecond set is non-uniform whereby collimator cells at differentlocations in the collimator are of different sizes.

According to some embodiments, the collimator cells are square.

According to some embodiments, the collimator cells are rectangular.

According to some embodiments, the septa in the first and/or second setsare of non-uniform thickness.

According to some embodiments, the septa of the first and second setsare of different lengths.

According to some embodiments, the septa of the first and/or second setsare respectively of varying lengths.

According to some embodiments, the detector element is pixilated.

According to some embodiments, the septa are positioned at the bordersof each pixel, whereby the collimator cells are aligned with thedetector pixels with one pixel per collimator cell.

According to some embodiments, the spacing of the septa of the firstand/or second sets is greater than the pixel pitch.

According to some embodiments, the spacing of the septa of the firstand/or second sets is smaller than the pixel pitch.

According to some embodiments, the septa of the first and/or second setsare respectively not parallel to each other.

According to some embodiments, the collimator cells are of differentsizes and are also of different in size from the size of the detectorelement pixels.

According to some embodiments, the pixels are arranged in a squarematrix or a rectangular matrix.

According to some embodiments, the detector element is a single crystalscintillator and is optically coupled to an array of PMTs.

According to some embodiments, the detector element is an array ofdirect conversion semiconductor detectors.

According to some embodiments, the detector element is configured as anarray of SiPMs.

According to some embodiments, the detector elements are formed of CZTor LYSO.

According to some embodiments, the septa are formed of tungsten.

According to some embodiments, the septa are configured to block lessthan 50% of 511 Kev radiation passing through at a thickness dimensionthereof.

According to an aspect of some embodiments of the invention, there isprovided a collimator for attachment to a Nuclear Medicine (NM) imagingsystem detector element responsive to gamma ray photons to provide anelectrical output signal, formed by first and second sets of septaextending in two directions that intersect to define an array ofcollimator cells, in which the septa are moveable to change the geometryof the collimator to increase and decrease the spatial resolution of thedetector unit.

According to an aspect of some embodiments of the invention, there isprovided a collimator for attachment to a Nuclear Medicine (NM) imagingsystem detector element responsive to gamma ray photons to generate anelectrical output signal, having a body defining first and second setsof septa extending in two directions, in which a thickness and/or heightof the septa in one direction is different from a thickness and/orheight in another direction.

According to an aspect of some embodiments of the invention, there isprovided a detector unit for a Nuclear Medicine (NM) imaging systemhaving a detector element responsive to gamma ray photons having energyin the range of about 40 KeV to 511 KeV, and a collimator configured topermit use of the detector for both PET and SPECT imaging.

According to some embodiments, the detector element is a scintillatorand the detector unit further includes an array of PMTs opticallycoupled to the detector element to provide electrical output signals.

According to some embodiments, the detector element is a directconversion element operative to generate an electrical output signal inresponse to impingement of a gamma ray photon.

According to some embodiments, an electronic signal processor isselectably operable to reconstruct a PET image or a SPECT image oroperable to simultaneously reconstruct PET and SPECT images.

According to some embodiments, the circuitry is configured toselectively process a signal from the detector as a single photon eventor as a coincidence event.

According to some embodiments, the geometry of the collimator isadjustable to provide high and low spatial resolution.

According to some embodiments, the collimator septa are formed of amaterial that absorbs PET energy photons with an efficiency of about 50percent or less of its efficiency of absorbing SPECT photons.

According to some embodiments, the collimator septa are formed oftungsten.

According to an aspect of some embodiments of the invention, there isprovided a method of imaging using an N-M tomography system, in whichboth PET and SPECT data from a target area are simultaneously collectedusing a single set of detectors.

According to some embodiments, the geometry and/or position of detectorheads forming a detector array is adjusted relative to the target area.

According to some embodiments, multiple radiopharmaceutical tracersproviding photon emissions having different energies are employed.

According to some embodiments, the detectors are comprised of radiationdetector elements and collimators that are adjustable to providedifferent degrees of spatial resolution.

According to an aspect of some embodiments of the invention, there isprovided a method of imaging using an N-M tomography system having acollimated detector unit, in which the configuration of the collimatorsepta are adjusted to provided a desired degree of spatial resolution;and SPECT imaging is performed using an algorithm that assumes acollection angle and a detection probability map according theadjustment of the collimator configuration, or PET imaging is performedtaking account of a detection probability resulting from the adjustedvariable septa geometry such that the probability is factored into theimage reconstruction.

According to an aspect of some embodiments of the invention, there isprovided a method of imaging using an N-M tomography system having aplurality of rotating detector units, in which the rotation of thedetector units is adjusted to provide a desired degree of spatialresolution and/or bore size and SPECT imaging is performed using analgorithm that assumes a collection angle and a detection probabilitymap according the adjustment, or PET imaging is performed taking accountof a detection probability resulting from the adjustment such that theprobability is factored into the image reconstruction.

According to some embodiments, there is provided an N-M tomographysystem in which the detector heads are formed by a detector elementresponsive to gamma ray photons to provide an electrical output signaland a collimator having first and second sets of septa extending in twodirections that intersect to define an array of collimator cells thatsepta are moveable to change the geometry of the collimator to increaseand decrease the spatial resolution of the detector unit.

According to some embodiments, the detector heads are formed of adetector element response to gamma ray photons having energy in therange of about 40 KeV to 511 KeV and a collimator configured to permituse of the detector for both PET and SPECT imaging.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions.

Optionally, the data processor includes a volatile memory for storinginstructions and/or data and/or a non-volatile storage, for example, amagnetic hard-disk and/or removable media, for storing instructionsand/or data. Optionally, a network connection is provided as well. Adisplay and/or a user input device such as a keyboard or mouse areoptionally provided as well.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1A is an exemplary schematic illustration of major externalfeatures of a N-M tomography system;

FIG. 1B is an exemplary schematic illustration of major externalfeatures of another commercially available SPECT system configuration;

FIG. 2A is an exemplary schematic illustration of the layout of adetector unit, for example, an Anger camera for a SPECT system;

FIG. 2B is an exemplary schematic illustration of an electronicsubsystem for z PET system;

FIGS. 3A-3D illustrate the concept of modularity of a detector headarray in an N-M tomography system in the context of an exemplary SPECTor PET system according to some embodiments of the invention;

FIGS. 4A and 4B illustrate upgrading an as-built single-mode PET orSPECT system into a dual-mode SPECT+PET system according to someembodiments of the invention;

FIG. 5 is an enlarged version of FIG. 4B according to some embodimentsof the invention;

FIGS. 6A and 6B illustrate the concept of variable bore size accordingto some embodiments of the invention;

FIGS. 6C and 6D illustrate advantages of variable bore size asimplemented according to some embodiments of the invention;

FIG. 6E is a side perspective view that illustrates schematically alinear motion mechanism according to some embodiments of the invention;

FIG. 6F is a side elevation of FIG. 6E;

FIG. 6G illustrates an exemplary mechanism for extending and retractinga detector head according to some embodiments of the invention;

FIGS. 7A and 7B illustrate a way to achieve variable bore size shown byway of example for a system operating in a PET mode according to someembodiments of the invention;

FIGS. 8A-8D illustrate another way to achieve variable bore size shownby way of example for a system operating in a PET mode according to someembodiments of the invention;

FIGS. 9A-9C illustrate yet another way to achieve variable bore sizeshown by way of example for a system operating in a PET mode accordingto some embodiments of the invention;

FIGS. 10A and 10B illustrate still another way to achieve variable boresize shown by way of example for a system operating in a PET modeaccording to some embodiments of the invention;

FIGS. 11A-11D illustrate still another way to achieve variable bore sizeaccording to some embodiments of the invention;

FIG. 12A illustrates an exemplary embodiment in which detector heads aretranslatable linearly on a gantry;

FIG. 12B illustrates an exemplary embodiment in which detector heads aretranslatable circumferentially on a gantry;

FIG. 13 illustrates benefits of continuous rotation of a detectorcarrier according to some embodiments of the invention;

FIG. 14 illustrates an arrangement for dynamically varying the bore sizeduring a scan;

FIG. 15 is a flow diagram illustrating use of an N-M tomography systemaccording to other embodiments of the invention;

FIG. 16 is a flow diagram illustrating use of an N-M tomography systemaccording to some other embodiments of the invention;

FIG. 17 illustrates the construction and assembly of a basic collimator;

FIGS. 18A-18C illustrate some exemplary (but non-limiting) ways to varythe resolution of collimators according to some embodiments of theinvention by lengthening or shortening the collimator septa;

FIGS. 19A and 19B illustrate exemplary (but non-limiting) ways to varythe resolution of collimators according to some embodiments of theinvention by sliding the septa laterally;

FIGS. 20A and 20B illustrate exemplary (but non-limiting) ways to varythe resolution of collimators according to some embodiments of theinvention by tilting and/or shifting the septa;

FIGS. 21A-21D illustrate resolution adjustment using an arrangement oflayered or vertically tandem collimator sub-units or parts, inaccordance with some embodiments of the invention;

FIGS. 22A-22D illustrate exemplary (but non-limiting) ways to vary theresolution of collimators according to some embodiments of the inventionusing adjustable shutters; and

FIGS. 23A-23C illustrate representative (qualitative) performance ofcollimator constructions and configurations according to someembodiments of the invention.

In conjunction with the following detailed description of variousaspects of some embodiments of the invention, it is to be understoodthat the invention is not necessarily limited in its application to thedetails of construction and the arrangement of the components and/ormethods set forth in the following description and/or illustrated in thedrawings and/or the Examples. The invention is capable of otherembodiments or of being practiced or carried out in various ways.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION IntroductoryOverview:

The present invention, in some of its embodiments, relates to apparatusand methods of tomography in the field of nuclear medicine (sometimesreferred to herein as “N-M tomography”), and more particularly to N-Mtomography systems as a whole, to detector configurations for N-Mtomography systems, and methods of using and/or upgrading such systemsand detector arrangements. In some embodiments, the invention alsorelates to methods of upgrading existing N-M tomography systems anddetectors.

In an exemplary embodiment of the invention, there is provided a singlesystem which is easily (e.g., field) upgradable and also capable ofdual-mode functionality, e.g., having the capability of selectablyoperating in either SPECT or PET mode (or both) without materiallysacrificing performance in either mode. In an exemplary embodiment ofthe invention, there is a provided a single detector that can beselectably operable in either a SPECT a PET mode with good resolutionand sensitivity.

In an exemplary embodiment of the invention, there is a provided asystem to be capable of operating simultaneously in both a SPECT and aPET mode.

Some embodiments of the present invention are directed to the issuesdiscussed above, and/or to other aspects of N-M tomography technology.

Aspects of some embodiments of the invention are equally applicable tosingle function SPECT and PET systems, and to dual function SPECT andPET systems, unless otherwise indicated.

An aspect of some embodiments of N-M tomography systems according to thepresent invention resides in detector systems comprised of multipledetector heads (for example, 3-18 heads), each head including multipleindividual detector elements (for example, 4-10 or more individualdetector elements). The detector units are arranged to form a boredefining a space within which the patient as a whole or a part of thepatient's body, i.e., an ROI, is examined.

In some of such embodiments, the detector heads are moveable (e.g.,during imaging and/or as a set up for an imaging session) relative tothe patient carrier and/or to each other and/or to the gantry in variousways allowing adjustment of the size and/or shape of the bore accordingto the particular ROI without obstruction or collision of adjacentdetector heads. In some embodiments, the systems are operable to adjustthe size and shape of the bore during a scan.

Optionally, rapid reconfiguration (e.g., faster than 2 hours, 1 hour, 30minutes, 10 minutes, 5 minutes per detector head) of the detector headsfrom one position and/or orientation to another for step and shootoperation is facilitated by a light-weight design of the detector headsand/or by counterbalancing.

Optionally, in some embodiments of the invention, the various degrees offreedom can also be implemented in a horizontal system, in which thepatient stands or sits, e.g., such that system axis (as well as the mainpatient axis) is vertical or at an angle to both vertical andperpendicular, and the gantry is relatively horizontal (or at an angle,such as between 20 and 80 degrees to the horizontal.

In some embodiments of the invention, the system provides single-modefunctionality, in which case, the detector heads are comprised of onlySPECT detectors, or only PET detectors. In some embodiments, the systemprovides dual-mode functionality, in which case, the detector systemincludes separate detector heads carrying PET and SPECT detectors, ordetector heads that include both PET and SPECT detectors.

Optionally, the detector heads are constructed of detectors that canselectably operate as either PET or SPECT detectors or both SPECT andPET detectors simultaneously. Optionally, such detector heads mayinclude the detector elements, collimators, and/or circuitry that canoperate, for example, in single photon detection mode and in coincidencemode. Optionally, timing circuitry is provided in a detector head andcoincidence circuitry in a more central location such as a CPU.Alternatively, coincidence circuitry as well is provided in associationwith a detector head (e.g., module, possibly a head on an arm),optionally the head receiving data on other detections from anotherhead, for example, via a system bus interconnecting heads.

In some embodiments, PET detector heads are T-shaped, or L-shaped (e.g.,with wider part facing the bore), and SPECT detector heads are rod orI-shaped. The detector head include elongated stems that serve as theaxis for extension/retraction and the detector elements arrayed on theends of the stems in various polygonal configurations including squareor rectangular or triangular, or in circular or arc-shapedconfigurations, or combination thereof. Optionally, in rest positions,the detector elements extend longitudinally in the direction of thesystem axis.

Optionally, the detector heads are comprised of a single detectorelement or a plurality of detector elements forming a pixilateddetector.

For convenience, the terms “detector arrangement” or “detector system”or “detector unit” will sometimes be used in reference to multiple andsingle-detector heads, and to heads that carry SPECT and/or PETdetectors, and without distinction as to detector head shape.

An aspect of some embodiments of N-M tomography systems according to thepresent invention resides in variable-geometry detector systems in whichthe detector heads are moveable relative to each other and to the gantryin various ways to improve sensitivity and spatial resolution forimaging different ROIs.

An aspect of some embodiments resides in variable-geometry detectorsystems in which the detector units are non-uniformly arranged on thegantry with (possibly) large gaps between them wherein the adjustabilityof the detector heads still provides full 360 degree detector coveragepossibly without loss (or with improvement) of sensitivity and/orspatial resolution for differently sized and shaped ROIs and/or atdifferent positions along the body of a subject under examination. Insome embodiments of the present invention, such configurations possiblyreduce the overall number of detectors needed for a given level ofspatial resolution and sensitivity, and thus reduce the overall systemcost.

The variable geometry features may allow trading off cost and imagingefficiency. The detectors represent a major component in the cost of asystem, so being able to vary the bore size and shape according to thesize of the patient and/or the location and/or the size of the ROI, mayallow generating good images with a smaller number of detectors thanwould be needed in conventional fixed-geometry systems. However, for alarger ROI or a larger patient, longer imaging times may result fromreducing the number of detectors. However, this may allow a majority ofstudies to be performed fast and/or at a lower cost.

In some embodiments, the detector heads are extended and retracted by alinear extension and retraction mechanism. Optionally, for operation ina PET mode, the detector heads are extended individually or in opposedpairs. Optionally, the opposed pairs are diametrically opposed.

Optionally, multiple detector heads are mounted on and/or moved togetheralong a single arm for in-out and/or lateral motion.

Optionally, at least some of the detector heads are not moved and datafor imaging is optionally collected from both moved and unmoved detectorheads.

Optionally, a detector head includes electronic circuitry that supportsmore than one set of separate detector elements. For example, onedetector head may include circuitry to support processing of signalsfrom two detector heads which are connected by a data cable. In anotherexample, a detector head is set up to support multiple types ofdetectors and/or collimators, which may be selectively mounted thereon.Optionally, an RFID code or other machine readable indicator on thedetector and/or collimator serve to indicate to the processing circuitry(e.g., in arm or in main machine) which type of detection is availableand/or to guide data acquisition, acquisition planning and/orreconstruction, according to the ability of the detector. Optionally,the indicator, or a different storage location includes a table or a setof parameters matching the type of detector and parameters or softwarefor using the detector.

If the size and/or shape of the individual detector heads; (particularlybut not exclusively in the case of PET detector heads) do not permitsufficient extension to reduce the bore size to a desired degree withoutcollision or interference between adjacent detector heads, severaloptions are available in accordance with some embodiments of theinvention. In some embodiments, only some of the detector heads areextended. For example, every other detector head (i.e., one-half thetotal number of detector heads) are extended, and the others remainun-extended. Optionally, the un-extended detector heads are not usedduring the scan. Optionally, the un-extended detector heads and theextended detector heads are used during the scan.

Alternatively, the angular orientation of at least some or all of thedetector heads may be varied relative to the axes of extension of therespective detectors to increase the amount that the detectors can beextended without collision. This can be advantageous since allowing agreater range of bore size adjustability can, potentially, betteraccommodate differently sized and shaped ROIs and ROIs at differentlocations along the subject's body. Optionally, the angular orientationof the detector heads can be varied either in pairs or individually.Further optionally, one or more of the heads that are connectable to thesame arm can be angularly oriented independently or differently thanother heads.

According to some embodiments of the invention, at least some of thedetector heads are angularly adjustable to a desired orientation in aplane perpendicular to a longitudinal axis of the individual detectors.(This feature is generally referred to below as “rotation” relative tothe longitudinal axis.) Typically, but not necessarily, depending on theshape of the detector head, the longitudinal axis corresponds to an axisof extension and retraction.

Optionally, the rotational orientation can be varied from a restposition by up to 90 degrees (or more) such that for some angles anddetector dimensions, some or all the detector heads overlap. This maynot only facilitate extending at least some of the detectors to obtain asmaller bore size, but may also result in obtaining good 360 degreecoverage with a smaller number of detectors.

According to some embodiments of the invention, at least some of thedetector heads are angularly adjustable to a desired orientation that isnot in a plane perpendicular to a longitudinal axis of the detectorheads. (This feature is generally referred to below as “tilting”relative to the longitudinal axis.) Optionally, the desired tilt angleis achieved by rotation around an axis parallel to an axis of elongationof the overall system.

According to some embodiments of the invention, increased reduction inbore size is achieved by axially spacing the detector heads on thegantry. Optionally, the detector heads are axially spaced on one side ofa single ring on the gantry. Alternatively, the detector heads arearranged on opposite sides of a single ring. Alternatively the detectorheads are arranged on one or two sides of two separately spaced ringscomprised in the gantry.

Optionally, for example, in the case of a PET system, adjacent detectorhead pairs are located at different axial spacing to avoid interferencebetween adjacent detector heads. Extension in combination with one ofthe orientation options may allow a reduced number of detector headsand/or detector head-pairs in the detector array while still providinggood 360 degree coverage. This can significantly reduce the cost of thedetector array. In addition, reducing the number of detector pairsallows the gantry to be constructed with open spaces between thedetectors along the periphery of the gantry, which facilitatesupgradability as described below.

It should also be noted that extension of the detector heads to create asmaller bore size generally has the effect of positioning the detectorheads closer to the ROI, consequently, each detector head subtends alarger solid angle around the ROI, and is able to collect more photonsemitted from the ROI. The result is that overall system sensitivity maybe improved and/or a smaller number and/o size o detectors may be used.Such approaching is optionally used in PET and/or in non-tomographicimaging modes, such as planar imaging.

Positioning the detector heads closer to the ROI may also improve thespatial resolution by decreasing nonlinearity as discussed below.

An aspect of some embodiments of the invention resides in N-M tomographysystems in which the detector heads include detector positioningarrangements that are operable to extend and retract and to angularlyorient desired combinations of detectors. Optionally, the positioningarrangements are comprised of a first mechanism associated with eachdetector head to effect extension and retraction, and a separatemechanism to angularly orient the detector heads. Optionally, a singlemechanism for extending and retracting and angularly orienting thedetector heads is associated with each of the detector heads.

An aspect of some embodiments of the invention resides in N-M tomographysystems in which the individual detector heads are translatable, e.g.,movable laterally or circumferentially on the gantry, continuouslyduring the scan and/or in steps so that the spacing between the detectorheads can be changed. For example, each detector head can be translated5, 10, 15, or 20 degrees, or a greater or lesser or intermediate amountsfrom a nominal equally spaced configuration. Optionally, each detectorhead is movable independently from the others, or jointly with one orsome or all of the others. Optionally, in combined PET—SPECT dualfunction systems, either the PET and SPECT detector heads, or both arecircumferentially movable. Optionally, the PET and SPECT detectors arelocated at (and/or attached to the gantry at) different axial positions,for example, on one or both sides of a single rotor disc, or on separaterotor discs, optionally to provide mechanical clearance.

As used herein, the term “circumferential movement” refers to rotationof the gantry ring or rings, and also includes movement of the detectorheads on the gantry. Likewise, circumferential movement includestranslational movement of each of the detector heads individually, i.e.,independently, or in groups within the ring, and/or movement of anentire ring relative to other rings.

Translation of the detector heads can be advantageous in varioussituations. For example, on a gantry having a small number of detectorheads e.g., as purchased by a customer with limited resources, there maybe large gaps between detector heads. Similarly, when the system isconstructed of a segmented gantry (optionally with each segmentincluding more than one detector head) the segments can be movedradially outward so the bore is expandable when necessary (e.g. toaccommodate an obese patient). In either case, the gaps between theheads may degrade data acquisition. Circumferential movement effectivelyshifts the gaps and allows capturing data from the gaps to complete themissing views.

Optionally, according to some embodiments, the gaps can be closed byrotation of the gantry or alternatively by relative rotation among thedetectors, for example, by translating the detector headscircumferentially on a single-ring gantry, of in the case of amultiple-ring gantry, by rotating one or more of the rings relative tothe others, to complete the full set of angles between pairs ofdetectors. Effectively, the bore is expanded radially and gaps arecreated, the circumferential motion (if any) allows the gaps to befilled. Optionally, the system is constructed so translation can occurduring a scan or in steps for step and shoot operation.

In some embodiments, gaps (e.g., between 1 and 30 degrees, for example,between 2 and 10 or 20 degrees) in an axial direction and/or in acircumferential direction are tolerated. Optionally, reconstructionweights (e.g., for sensitivity) certain directions according to thepresence and/or size of gaps therein.

A feature of some embodiments of N-M tomography systems according to thepresent invention resides in a gantry that is slidable and/or rotatablelaterally, to capture an image from a “body slice” which is orthogonalor not orthogonal to the main body-axis, and/or move along the body ofthe patient, for example to capture “slice by slice”.

In an exemplary embodiment of the invention, a controller canselectively move one or more of the detector heads alone, independently,in groups and/or separately but optionally in synchrony with otherdetector heads.

An aspect of some embodiments of N-M tomography systems according to thepresent invention resides in matching the detector bore to the ROI bymaking the extension and/or angular orientation adjustable while thescan is being performed.

A related aspect of some embodiments resides in detector arrays per sehaving detectors that are extendable and/or angularly adjustable duringa scan.

By way of summary, adjustment of the size and shape of the bore as wellas improved sensitivity and resolution is achieved by optionallyproviding one or more of the degrees of freedom listed below.

For the gantry (e.g., in either SPECT or PET operation unless otherwisenoted):

-   -   (a) The gantry can be a full circle, or a partial circle.    -   (b) The gantry can include one ring or multiple rings. The        planes defined by the rings may be parallel to each other, or        non-parallel. In a PET mode, the gantry may be rotated        circumferentially, either continuously during generation of data        for a slice, or in steps (referred to herein as “step and shoot”        operation).    -   (c) The gantry may move vertically relative to the system axis,        or can be tilted (e.g., using a motor and/or a gear) to one or        more non-vertical orientations, and/or to one or more        orientations that are non-orthogonal to the system axis to        obtain views that can overcome attenuation or other obstruction        or scatter, or to obtain additional complementary information        that helps stabilize the image reconstruction process.    -   (d) Optimally, some or all the adjustments mentioned above can        be performed manually. Optionally or alternatively these        adjustments can be motorized and controlled by the system        controller.

For the Detector Heads (e.g., in SPECT or PET operation):

-   -   (e) Some or all the heads can move in and out, radially, i.e.        extend and retract, to increase or decrease the bore size. The        extension/retraction can be the same for all the detector heads,        or may be different depending on the location of a particular        ROI in the body;    -   (f) The detector heads can move laterally relative to each other        and/or to the gantry. The movement of the detector heads can be        linear or along a non-straight-line path, for example, a curved        or piece-wise linear paths elected to avoid or reduce collisions        between adjacent detector heads;    -   (g) The detector heads can be rotated around an axis which is        substantially orthogonal to the system axis, e.g., around the        axis of extension/retraction;    -   (h) The detector heads can be tilted in one or more planes        relative to the axis of extension/retraction. Tilting can be        effected by rotating the head around an axis which is        substantially parallel to the system axis, or by rotation around        an axis which is non-parallel to the system axis;    -   (i) The system controller can be programmed to move the detector        heads in a manner that prevents collision during movement, for        example, by calculating dynamics to predict a collision and slow        down movement as needed. Optionally or alternatively, sensors        are provide (e.g., IR or ultrasonic proximity sensors) at the        sides of the detectors, to detect imminent collisions.

An aspect of some embodiments of the invention resides in PET systems orin dual purpose systems operating in a PET mode, in which the detectorheads are arranged around less than the full 360 degrees of the gantryand in which the gantry rotates, e.g., as in typical SPECT systems.Optionally, the scan is performed at a succession of axial slices.

Optionally, the rotation is continuous at each axial position.Alternatively, the gantry rotates in steps of less than 360 degrees andis temporarily stationary at each step. Optionally the size of the stepsis based on the size of the gaps between the detectors. Optionally, thecontinuous or step-wise gantry rotation during a PET scan is repeatedfor each of a succession of axial slice positions. Optionally, inembodiments in which PET detectors are mounted on more than one axiallyspaced rotor on the gantry, the rotational speed of each rotor may bethe same or different.

An aspect of some embodiments of N-M tomography systems according to thepresent invention resides in the detector arrays being positionable atone or more desired distances from the patient's body. Optionally,positioning may be done before or during a scan, optionallycontinuously, or between axial slices.

An aspect of some embodiments of the present invention resides in N-Mtomography systems that include proximity detection capability toprevent contact between the detector array and the body of the patient.Optionally, proximity detection capability is provided by contactsensors, or by acoustic sensors, or by IR sensors, or by opticalsensors.

An aspect of some embodiments of the present invention resides in N-Mtomography systems that include proximity detection capability toprevent contact between a detector and an adjacent detector and/or toprevent contact and/or pinching of body parts between detectors (e.g.,if patient moves his harm into harm's way, for example, proximitydetection capability is provided by contact sensors, or by acousticsensors, or by IR sensors, or by optical sensors.

In an exemplary embodiment of the invention, contact sensors areacceptable, because detector motion uses low forces, detectors arelow-weight and/or covered with a soft layer and/or detectors can bequickly stopped (e.g., using brakes or suitable motor/actuator action).

An aspect of some embodiments of N-M tomography systems according to thepresent invention resides in detector arrays that are allowed to makecontact with the patient's body, but with such a low contact forceand/or velocity that injury to the patent does not occur. In some suchembodiments, the detector arrays are counterbalanced on the linearactuator arms so the force needed to extend the detector arrays isacceptably small (e.g., using a stepper motor which generates up to 3 Kgforce only). Optionally, the extension force is small enough that thepatient can move the detector array away from his or her body.Optionally, the actuator allows such back driving, for example, usinggears which can be back driven or by a linear actuator which can beoverridden by patient applied force. Further, because of the small massof the individual detector heads, impact with the body is optionallysmall. Also because of the low mass, the velocity is easily reducedbefore impact.

An aspect of some embodiments of the present invention resides in N-Mtomography systems having modular and/or scalable detector arrays. Arelated aspect of some embodiments of the invention resides in modularor scalable detector arrays per se.

Modularity can allow initial assembly of N-M tomography systems havingdetector arrays with a desired number of individual detector headsaccording to a particular customer's initial needs, and facilitatessubsequent upgrading. This can give a customer the option, both at thetime the system is acquired, and/or at the time of an upgrade, to tradeoff cost versus quality, for example, as described herein. For example,three, four, six, eight, twelve, or an intermediate or greater number ofdetector heads can be provided initially, and more added later as needsand/or financial resources of the customer change. Optionally oralternatively, detectors can be replaced with different and/or betterdetectors.

In an exemplary embodiment of the invention, either by way ofidentifying data provided by the added detector heads, or by informationprovided manually to the system controller, the software knows whatdetectors have been installed, and the information can be used in thecourse of data acquisition and/or image reconstruction.

In some embodiments, the detector arrays as originally assembled are forsingle-mode SPECT or PET systems. Optionally, the detector arrays asoriginally assembled include both SPECT and PET detector heads allowingdual-mode system functionality.

Optionally, as part of an upgrade, existing detectors may be replaced bybetter or improved detectors, for example, having faster circuitry,larger detection area, and/or better energy and/or spatial resolution.Optionally, detectors can be added and/or replaced when upgrading eithera single-mode or a dual-mode system to improve the functionality of thesystem.

Optionally, an upgrade can convert a single-mode system into a dual-modesystem. Optionally, x-ray CT capability may also be provided in newand/or upgraded single and dual-mode systems.

Optionally, features contributing to modularity according to someembodiments include, without limitation, one or more of the following:

-   -   (a) The gantry which carries the detector array is rotatably        mounted in the initially assembled system, whether it is        single-mode (SPECT or PET) or dual mode;    -   (b) Connection of the detectors to the coincidence processing        electronics is through a rotatable coupling arrangement;    -   (c) The image reconstruction electronics and/or the coincidence        processing electronics are adapted to recognize the number and        type (SPECT or PET) of detector heads in the detector array,        either automatically, or by programming at the time of assembly        or upgrading of an existing system;    -   (d) The detector heads provide identification as to connection        and/or type for auto-recognition by the electronics sub-systems;    -   (e) A rotary drive system is easily installed as part of an        upgrade;    -   (f) The electronic sub-systems are modular to facilitate        converting a single-mode system into a dual mode system.

Various ways to implement some of the above-described features will beapparent to those skilled in the art, especially in view of theexemplary methods of implementations are described below.

In some embodiments of the invention, PET detectors are used, i.e., thatare used for detecting high energy photon pairs travelling in oppositedirections by identifying the locations in which 2 photons hit 2detectors simultaneously (up to photon travel time and detection time),thus enabling identifying the orientation from which the photon has beenemitted at a much finer precision. For example, such embodiments allowdetection along a line with a width of about 4-6 mm, taking into accountthe distance of the positron traveling until annihilated (about 2-4 mm),and the pixel width in each detector (for example 2-3 mm)

Optionally, however, in some embodiments, acquisition of PET (highenergy) photons can be done without coincidence detection using “SPECTmethodology” (detecting each photon separately) by providing a thickcollimators and detector heads capable of detecting typical PET andSPECT photons.

Furthermore, coincidence detection circuitry can optionally includetime-of-flight analysis circuitry to determine where along the estimatedemission line the positron was emitted, for example at a longitudinalresolution of about 1-5 cm, for example 2-3 cm. For example, an optionalsystem clock can be shared by the detector heads. As an optionalalternative, processing is in central location which pre-calibratestravel time from each detector of signals.

In some embodiments, the electronic circuitry connected to some or allof the detector heads includes one or more of the following optionalcapabilities:

-   -   (a) Photon characterization by energy level (for example, within        all the range between 40 Key and 511 Key or more);    -   (b) Detection time with resolution sufficient to determine        coincidence with photon detection in another detectors,    -   (c) Detection time with resolution sufficient to determine time        of flight for obtaining high longitudinal resolution along the        detected coincidence line;    -   (d) Detection of count rate in case of high flux of photons, for        example when a high intensity radiation source is activated such        as X-ray source;    -   (e) Optionally, the electronics include multiple separate        channels that allow independent amplification and front-end        processing for each detector or small group of detectors (e.g.,        1-5 detectors) and/or a small number of pixels (e.g., between 10        and 1000, for example 100 pixels). A potential advantage is that        malfunction of one or more pixels or detectors and/or blinding        of one or more pixels or detectors by a “hot spot” (high        intensity source) desirably will not prevent other detectors        from properly function and detect photons emitted from other        regions, for example as described in US patent publication        2008-0230702-A1.    -   (f) Optionally, the processing channels may also be modular, for        example, being field replaceable and/or include their own        housings.

An aspect of some embodiments of the invention pertains to a method ofusing N-M tomography systems including detector arrays having some orall the adjustability features described herein that involves preparingthe patient in the normal manner, setting up the system for anexamination by adjusting the bore size and/or shape, then axiallyscanning the region of interest, optionally axially. Optionally theadjustment is achieved by extending at least some of the detector headsand, if necessary, angularly orienting at least some of the detectorheads in the detector array according to the size and/or shape of theROI and/or the axial position of the ROI along the body of the patient.Optionally, the angular orientation is adjusted by rotation of at leastparts of some of the detector heads. Optionally, the angular orientationis adjusted by tilting at least some of the detector heads.

Optionally the adjustments can be made during scanning, for example inresponse to change in body cross-section, imaging mode.

Optionally, the method applies to single and dual-purpose systems.

In a system providing dual-mode functionality, the method furtheroptionally includes selectable operating the system in a SPECT or in aPET mode.

In some embodiments, the method involves providing a plurality ofadditional detector units that include built-in mechanisms forextension/retraction and angular orientation. Optionally, the additionaldetector units are mounted in alternating relationship with the existingdetector units on the gantry. Optionally, if the pre-existing systemdoes not provide automatic detection of the number and type of detectorunits, the method further includes adding automatic detection orprogramming the emission detector subsystem according to the number andtype of detector units in the upgraded system.

Optionally, the position adjusting arrangement provided is operable toextend and retract the detector units, and/or to alter the angularorientation of the detector units by rotation and/or tilting at leastsome of the detector units. Optionally, the position adjustingarrangement includes separate extension/retraction and angularorientation mechanisms. Optionally, each added detector unit includesits own mechanisms for extension and retraction, and/or for rotating ortilting.

Optionally, the detector units of the upgraded system are mounted sothat some of them in the upgraded detector array are axially spaced fromothers, but on one side of a gantry ring. Optionally, some of thedetector units in the upgraded detector array are mounted on oppositesides of a detector carrier ring. Optionally some of the detector unitsin the upgraded system are mounted on axially offset detector carrierrings.

It should be understood, that upgradability as described herein isfeasible in systems that do not contain scalable detector arrays, butthe benefits may be attenuated since the entire preexisting detectorarray may need to be replaced, and the emission processing and/or imagereconstruction sub-systems may have to be reprogrammed or even replaced.

An aspect of some embodiments of the present invention resides in N-Mtomography systems in which the detector units are capable of respondingto photons in a range of energies including both PET and SPECT ranges.Optionally, the detectors and associated emission data processingsystems are selectably responsive to PET or SPECT photons, or,simultaneously responsive to PET and SPECT photons to generate orreconstruct visual 3D images of regions of interest (ROI) of a patientbeing examined.

In such embodiments, and also in other exemplary embodiments describedherein, parts of the data processing systems are optionally containedwithin or mounted on the detector units.

In exemplary embodiments described herein, the radioactive emissiondetector is comprised of a scintillator optically coupled to an array ofphotomultipliers. Alternatively, the detector is a direct conversionsemiconductor array, or a silicon photomultiplier (SiPM) (see: Roncaliet al., supra).

Optionally, in the exemplary embodiments described herein, the emissiondetector elements are pixilated. Alternatively, at least some detectorelements are non-pixilated.

Optionally, in the exemplary embodiments described herein, the emissiondetector elements are formed of a known material including, but notlimited to, Lutetium Oxyothosilicate (LSO), Lutetium YittriumOxyothosilicate (LYSO), Cadmium Zinc Telluride (CZT), Cadmium Telluride(CdTe), Cesium Telluride (CsTe), Cesium Iodide (CsI), or of any othersuitable and desired material presently known or hereafter discovered orcreated.

An aspect of some embodiments of the present invention resides indetector units including collimators that permit selectable operation ineither a PET or SPECT mode, optionally without physical reconfigurationthereof. Optionally, the detector units are operable to simultaneouslyproduce PET and SPECT images.

Optionally, in such embodiments and in other exemplary embodimentsdescribed herein, the collimators are formed of a material thateffectively blocks photons having energy in the range used for SPECTimaging, but is relatively transparent to photons having energy in therange used for PET imaging. Optionally, the material forming thecollimators blocks no more than about 50% of incident PET photons.Optionally, the collimators are formed of Tungsten, or Tungsten Carbideor Lead or Gold or depleted Uranium or a combination of these materials.Optionally, the amount of blocking for PET detection is selected so thatat angles or directions where a higher sensitivity is desired, there isless blocking.

An aspect of some embodiments of the present invention resides indetector units including collimators having adjustable geometry thatpermits changing the spatial resolution of the detectors. Optionally,such adjustment permits the detector units to be selectably operatedeither as PET or SPECT detectors, or optionally to simultaneouslyproduce PET and SPECT images.

Optionally, according to some embodiments, the collimator geometry canbe varied in one or more of the following ways:

-   -   a) increasing the length of the septa forming the collimator        cells (the term “length” referring to the height of the        collimator perpendicular to the plane of the detector module);    -   b) increasing the spacing of the septa (i.e., the pitch) in one        and/or both directions relative to a surface of the detector        element (optionally, by removing one or more septa);    -   c) tilting some of the septa in one and/or both directions        relative to a surface of the plane of the detector element;    -   d) increasing or decreasing the pitch of the septa (i.e. the        distance between adjacent septa walls) in one and or both        directions relative to a surface of the detector element: the        pitch can be decreased, for example, by forming the collimator        of two or more relatively moveable parts parallel to the plane        of the detector element;    -   e) forming the collimator with a shutter to adjust the effective        size of the area exposed to incoming photons: optionally the        shutter is slidable or tiltable or in the form of an iris.

Optionally, the detector element has a planar surface relative to whichthe septa are moveable.

In some embodiments, these modifications are carried out while thecollimator is attached to the detector. In some embodiments, thecollimator is removed, modified and reattached. In some embodiments, thereconfiguration is provided in a laboratory and/or during manufacture.Optionally, the collimator-detector pair is pre-configured at multiplecollimator states.

In an exemplary embodiment of the invention, a pressure clamp and/or ascrew clamp and/or a locking rod (through the septa) mechanism are usedto hold septa in place relative to a body of the collimator.

An aspect of some embodiments of the present invention resides indetector units including collimators that have a first set of leaves ina first arrangement and a second set of leaves arranged to intersectwith the first set. Optionally, an average height perpendicular to adetector surface and/or an average thickness of leaves of the first andsecond sets are different. This may result in different viewing anglesin different direction and/or different amount of PET sensitivity indifferent directions.

An aspect of some embodiments of the present invention resides in amethod of imaging comprising using an N-M tomography system to collectPET and/or SPECT from a ROI using a single set of detectors. Optionally,PET or SPECT data are collected simultaneously. Optionally, when PET andSPECT data are collected simultaneously, PET and SPECT images aregenerated simultaneously using separate electronic subsystems.

In an exemplary embodiment of the invention, the amount of axial,circumferential and/or radial overlap at least 5%, 10%, 20%, 30% orintermediate or greater percentage of the dimension and/or detector areaof the detector head. Optionally or alternatively, the overlap is lessthan 90%, 80%, 50%, 40% or smaller or intermediate percentages thereof.

Rotation around an axis of a detector can be for example, 10, 30, 40, 70or smaller or intermediate degrees.

Optional Features of Some Embodiments of the Invention

The discussion below concerns further optional features of someembodiments of the invention according to the aspects of the inventiondiscussed above. It should be understood that one or more of thesefeatures may be combined with any embodiments of the detector units andmethods described herein above and/or below and/or provided with othersystems, unless otherwise clearly stated:

a. Multiple detector heads arranged around a gantry which are moveablerelative to the patient carrier and/or to each other and/or to thegantry to create a variable-geometry bore over a wide range of sizeswithout obstruction or collision of adjacent detector heads.

b. Adjustability of the bore geometry during a scan, optionally betweensteps of a step-and-shoot scan, or between axial positioning of thegantry for acquisition of data for a succession of axial slices.Optionally, adjustment may be performed both between step-and-shootpositions and between axial positions, or “on the fly” according to apre-set program for a specific patient scan, both during scan or in asequences scan scenario.

c. Rapid reconfiguration of detector geometry facilitated bylight-weight design of the detector heads and/or by counterbalancing thedetector units.

d. Variable bore geometry implemented in conventionally configuredsystems (with the patient lying on a horizontal carrier) and alsosystems, in which the patient stands or sits such that system axis (aswell as the main patient axis) is vertical, and the gantry is relativelyhorizontal.

e. PET detector heads that are T-shaped, or L-shaped, and SPECT detectorheads that are rod or I-shaped.

f. Detector heads including elongated stems that serve as an axis forextension/retraction with the detector elements arrayed on the ends ofthe stems in various polygonal configurations including square orrectangular or triangular, or in circular or arc-shaped configurations,or combination thereof. Optionally, in rest positions, the detectorelements extend longitudinally in the direction of the system axis.

g. Detector heads that are comprised of a single detector element or ofa plurality of detector elements, for example, pixilated detectors. Forconvenience, the terms “detector arrangement” or “detector system” willsometimes be used in reference to multiple and single-detector heads,and to heads that carry SPECT and/or PET detectors, and withoutdistinction as to detector head shape.

h. Variable-geometry detector systems in which the detector units arenon-uniformly arranged on a gantry with large gaps between them whereinthe adjustability of the detector heads still provides full 360 degreedetector coverage without loss of sensitivity and spatial resolution fordifferently sized and shaped ROIs and at different positions along thebody of a subject under examination. In some embodiments of the presentinvention, such configurations possibly reduce the overall number ofdetectors needed for a given level of spatial resolution andsensitivity, and potentially reduce the overall system cost.

i. Detector heads that are extended and retracted by linear actuators.Optionally, in a PET mode, detector heads that are extended individuallyor in opposed pairs. Optionally, the opposed pairs are diametricallyopposed. Optionally, multiple detector heads are mounted on and/or movedtogether along a single arm for in-out and/or lateral motion.

j. Detector arrays in which some of the detector heads are not moved anddata for imaging is optionally collected from both moved and unmoveddetector heads.

k. Detector heads that include electronic circuitry that supports morethan one set of separate detector elements, e.g., of different typesand/or geometric location.

l. Variable geometry detector arrays in which the detector heads;(particularly but not exclusively in the case of PET detector heads) aremoveable in ways that permit bores small enough for efficient imaging ofrelatively small organs such as the brain, the throat or an extremity,without collision or interference between adjacent detector heads.Options include:

(i) making only some of the detector heads, for example, every otherdetector head extensible, (with the un-extended detector headsoptionally used or are not used during the scan,

(ii) making the angular orientation of at least some of the detectorheads adjustable relative to the axes of extension of the respectivedetectors to increase the amount that the detectors can be extendedwithout collision. Optionally, the angular orientation of the detectorheads can be varied either in pairs or individually. Further optionally,one or more heads that are connectable to the same arm can be angularlyoriented independently or differently than other heads,

(iii) making at least some of the detector heads rotatable or tiltable.It should also be noted that extension of the detector heads to create asmaller bore size has the effect of positioning the detector headscloser to the ROI, consequently, each detector head subtends a largersolid angle around the ROI, and is able to collect more photons emittedfrom the ROI, The result is that overall system sensitivity may beimproved.

m. making detector heads translatable, i.e., movable circumferentiallyon the gantry, continuously during the scan or in steps so that thespacing between the detector heads can be changed. For example, eachdetector head can be translated 5, 10, 15, or 20 degrees, or a greateror lesser or intermediate amounts from a nominal equally spacedconfiguration. Optionally, each detector head is movable independentlyfrom the others, or jointly with one or some or all of the others.Optionally, in combined PET—SPECT dual function systems, either the PETand SPECT detector heads, or both are circumferentially movable.Optionally, the detectors heads can be moved in one or more straightline segments or along a curve.

n. Locating translatable PET and SPECT detectors at different axialpositions, for example, on one or both sides of a single rotor disc, oron separate rotor discs to provide mechanical clearance.

o. Configuring the detector arrays so that gaps between then can beeffectively closed by rotation of the gantry. (As used herein, the term“circumferential movement” also refers to rotation of the gantry ring orrings, continuously and/or in steps) as well as by translating thedetector heads circumferentially on the gantry to complete the full setof angles between adjacent detector heads. Optionally, the system isconstructed so translation can occur during a scan or in circumferentialsteps (for step and shoot operation) or between axial positions.

p. Making the gantry slidable and/or rotatable laterally, to capture animage from a “body slice” which is orthogonal or not orthogonal to themain body-axis, and/or move along the body of the patient, for exampleto capture “slice by slice”.

q. Providing for selective movement of one or more the detector headsalone, independently, in groups and/or separately but in synchrony withother detector heads.

r. The gantry can be fully circular, or a partial circle, as well asother shapes.

s. The planes defined by multiple gantry rings may be parallel to eachother, or non-parallel.

t. The gantry may move vertically relative to the system axis, or can betilted to one or more non-vertical orientations (e.g., be mounted on amotorized axle), and/or to one or more orientations that arenon-orthogonal to the system axis to obtain views that can overcomeattenuation or other obstruction or scatter, or to obtain additionalcomplementary information that helps stabilize the image reconstructionprocess.

u. Gantry rotation is continuous at each axial position for both PET andSPECT imaging. Alternatively, the gantry rotates in steps, the size ofwhich is optionally based on the size of the gaps between the detectors.Optionally, in embodiments in which PET detectors are mounted on morethan one axially spaced rotor on the gantry, the rotational speed ofeach rotor may be the same or different.

v. Providing proximity and/or side detection capability to preventcontact between the detector array and the body of the patient.Optionally, proximity detection capability is provided by contactsensors, or by acoustic sensors, or by IR sensors, or by opticalsensors, or in any other suitable and desired manner

w. Permitting the detector heads to make contact with the patient'sbody, but with such a low contact force and/or velocity that injury tothe patent does not occur.

x. Counterbalancing the detector heads so the force needed to extend thedetector heads is acceptably and safely small. Optionally, the force issmall enough so that a patient can easily resist the force or can movethe detector array away from his or her body by hand if necessary. Theoptional small effective mass of the individual detector heads, allowsthe velocity to be easily reduced before impact.

y. The detector head counterbalancing mechanisms include adaptive motionfeedback capability for safety control and acquisition continuation ifthe detector head is touched or pushed back, for example, by thepatient.

z. Configuring the system so that the detector arrays are modular orscalable.

aa. Either by way of identifying data provided by the detector headsadded as part of an upgrade, or by information provided manually to thesystem controller, the system software is made aware of what detectorshave been installed, and the information can be used in the course ofdata acquisition and image reconstruction.

bb. The gantry which carries the detector array may be rotatably mountedin the initially assembled system, whether it is single-mode (SPECT orPET) or dual mode;

cc. The detector heads are connected to the processing electronicsthrough a rotatable coupling arrangement;

dd. The gantry is configured so it requires only minimum disassembly forupgrading, including to facilitate installation of a rotary drivesystem, thereby helping to permit on-site upgrading;

ee. The electronic sub-systems are modular to facilitate converting asingle-mode system to a dual mode system.

ff. Coincidence detection circuitry for PET imaging can includetime-of-flight analysis circuitry to determine where along the estimatedemission line the positron was emitted, for example at a longitudinalresolution of about 1-5 cm, for example 2-3 cm. For example, an optionalsystem clock can be shared by the detector heads. As an optionalalternative, processing is in central location which pre-calibratestravel time from each detector of signals.

gg. The electronic circuitry connected to some or all of the detectorheads includes one or more of the following optional capabilities:

-   -   (i) Photon characterization by energy level (for example, within        all the range between 40 Key and 511 Key);    -   (ii) Detection time resolution sufficient to determine        coincidence with photon detection in another detector,    -   (iii) Detection time resolution sufficient to determine time of        flight for obtaining high longitudinal resolution along the        detected coincidence line,    -   (iv) Detection of count rate in case of high flux of photons,        for example when a high intensity radiation source is activated        such as X-ray source.

hh. The electronics include multiple separate channels that allowindependent amplification and front-end processing for each detector orsmall group of detectors and/or a small number of pixels (e.g., between10 and 1000, for example 100), such that any malfunction of one or morepixels or detectors and any blinding of one or more pixels or detectorsby a “hot spot” (high intensity source) do not prevent other detectorsfrom properly function and detect photons emitted from other regions.Optionally, the processing channels may also be modular.

An aspect of some embodiments of the invention resides pertains to amethod of using an N-M tomography system including detector arraysoptionally having some or all the adjustability features describedherein. In some embodiments, both PET and SPECT imaging can beperformed, either sequentially or simultaneously. Optionally, the methodinvolves preparing the patient in the normal manner, setting up thesystem for an examination by adjusting the bore size and/or shape, thenscanning the ROI. Optionally the adjustment is achieved by extending atleast some of the detector heads and, if necessary, angularly orientingat least some of the detector heads in the detector array according tothe size and/or shape of the ROI and/or the axial position of the ROIalong the body of the patient.

In a system providing CT capability, the latter modality may optionallyalso be employed as part of a unified examination.

An aspect of some embodiments of N-M tomography systems according to thepresent invention resides in use of collimated detectors for PETimaging, with image reconstruction software that compensates for reducedoff-axis sensitivity resulting from photon absorption by the collimatorsepta, for example, by weighting of photon counts according to theirdirection of impact.

Exemplary System Features

Referring again to FIG. 1A, in some exemplary embodiments of theinvention, gantry 12 is mounted so that in addition to conventionalfunctionality by which it moves along the length of patient carrier 16(or vice versa) to capture emission data from a succession of “slices”orthogonal to the length of the patient's body, (i.e., the body-axis),it is optionally also constructed so it can slide transversely (e.g., ona rail) or tilt relative to the body-axis (e.g., the rail is mounted onan axle or an actuator is provided at either end to raise/lower a sideof the rail), to capture emission data in planes that are not orthogonalto the body-axis.

This capability can be advantageous, for example, if it is desired toacquire photons from viewing angles with less attenuation and scatterdue to bones (e.g. taking different viewing angles to overcomeattenuation and scatter by the ribs, etc), or to improve uniformity,quality and stability of the image reconstruction process by providingto the reconstruction algorithm information from additional viewingangles.

Modularity and Upgrade:

An aspect of some embodiments of the invention is modularity of thedetector arrays. Referring now to FIGS. 3A-3D, several implications ofthis are described in the context of a detector array 300 for a SPECTsystem according to some embodiments of the invention. In theillustrated context, it should be understood that modularity applies tothe design of detector array 300 such that it may be assembled from adesired number of individual detector heads 302 a, 302 b, etc. accordingto the specification of the customer.

FIG. 3A illustrates a detector array 300 with three individual detectorheads 30 (two being a practical lower limit in conventional systems).FIGS. 3B-3D respectively illustrate systems employing detector arrayshaving four, six and 12 individual detector heads 302. Exemplaryconfigurations of detector heads embodying features of the presentinvention are described below.

FIGS. 3A-3D also illustrate the trade-off resulting when increasingnumbers of detector heads are provided: performance is increased interms spatial resolution and/or sensitivity and/or speed of image dataacquisition for a range of ROI sizes and shapes and longitudinal (axial)locations along the body of the subject of an examination, but atpotentially significant increased cost for the detector heads.

Another implication of modularity is illustrated in FIGS. 4A and 4B: asystem can initially be assembled with a detector array 400 providingsingle-mode functionality (here illustrated as SPECT functionality) andcan conveniently be upgraded into a dual-mode system that provides bothSPECT and PET functionality.

Thus, FIG. 4A shows an as-built detector array 400 for a SPECT systemhaving six detector heads 402 with considerable open space 404 betweenthe individual detectors. FIG. 4B shows a detector array 406, forexample, after an upgrade of detector array 400. Here, six PET detectorheads 408 have been installed in alternating relationship with SPECTdetector heads 402. As will be understood, FIG. 4B can represent anas-built configuration or an upgraded detector array comprised only of12 PET detectors 408 or only of 12 SPECT detectors 402.

In an exemplary embodiment of the invention, a modularattachable/detachable component includes a housing suitable for exteriorviewing/environment, for example, with suitable paint and/or markings.In an exemplary embodiment of the invention, the attachment of thecomponent to the rest of the system includes separate electrical, dataand mechanical connectors. For example, plug-socket connectors may beused for power and data and a mechanical interlock used for mechanicalconnection. In some embodiments, the component will interlock to amovable part of the system. In an exemplary embodiment of the invention,two interlocks are used; one interlock providing alignment between thedetector and the system (or gantry), for example, a plurality of pinsmatching recesses and/or other geometries and a second interlockprovides interference to prevent retraction, for example, using one ormore screws, bolts or a locking rod. Optionally, a separate element(from the alignment geometry), such as a rectangular rod, is used toconvey forces between the system and a removable detector

Basic Detector Head Configurations

FIG. 5 shows exemplary details of SPECT and PET detector heads accordingto some embodiments of the invention mounted on one side of a singlegantry 500. SPECT detector heads 502 are shown as rod or I-shaped witharcuate, e.g., approximately cylindrical photon-collecting surfaces 510extending into the plane of the figure. PET detector heads 504 are shownas T-shaped with a stem portion 506 extending radially toward the systemaxis at the center of gantry 500 and a detector-carrying portion 508oriented tangentially to the periphery of the bore.

It will be appreciated that other external configurations are alsowithin the scope of some embodiments. For example, both the SPECT andPET detector heads can be L-shaped or otherwise have different shapes.Optionally, different detectors with different abilities have differentshapes and/or sizes.

Optionally and preferably, in some embodiments, the detector-carryingportions 508 of PET detector heads 504 are configured as plates thatextend in the direction of the system axis, i.e., into the plane of thefigure. This can be advantageous, for example in that it allows adesired degree of overlap between slices as the emission data is beingcollected, or wider slice width, to perform faster body scan (in thecase of multiple-slice scan).

The large detector head configuration for the PET detector heads 504 canbe advantageous because for optimal and uniform PET imagereconstruction, pairs of PET detectors sometimes need to cover as muchas possible of the whole 360 degrees of possible photon emission fromeach location in the ROI, and/or with a sufficient axial extent. Havinglarge detector surfaces for the PET detector heads may minimizes gaps inwhich coincidence lines are not covered and/or otherwise increasesensitivity, and may avoid reduced uniformity and/or sensitivity.

SPECT detectors, on the other hand can acquire 180 degrees around theROI in several positions at different times, so, in some embodimentsthey can be narrower and move to obtain the necessary viewing angles. Inan exemplary embodiment of the invention, care is taken, however, thatthe PET detectors are not so large that they obscure the view of theSPECT detectors when only the latter are in use.

As described herein, the dynamically variable geometry of the detectorheads according to some embodiments of the invention facilitatesoptimizing or near-optimizing the size and/or shape of both the PET andSPECT detectors. Potentially contributing to optimization is theplacement of the detectors on one or more gantry rings which can moveand operate independently as discussed herein. In an exemplaryembodiment of the invention, the system controller 30 (see FIGS. 2A-2B)may be programmed to plan and/or control the motion of the detectorsheads according to a desired (e.g., optimal) data collection from thedesired ROI, while optionally preventing adjacent detector heads fromcolliding and/or obscuring each other's field of view.

In some exemplary embodiments, the SPECT detector heads includedetectors (e.g., radiation sensitive elements thereof) that coveroverall about 1-40 cm, or 1-20 cm, for example, about 2-8 cm along thecircumferential dimension, i.e., along the gantry, for example about 4cm. In the axial direction, i.e., in the direction of the system axis,the SPECT detectors optionally cover overall about 10-40 cm, for exampleabout 12-32 cm, or 15-30 cm, or 16-28 cm, or about 16 cm, or about 20cm, or about 24 cm, or about 28 cm or intermediate sizes.

In some exemplary embodiments, collimators extend radially, i.e. towardthe patient's body a distance of a few cm (e.g. 1-20 cm, 1-4 cm, forexample about 2-3 cm).

In some embodiments, the SPECT detectors (with the collimator) of eachsuch detector head are rotatable, for example, around an axis parallelto the system axis. In some embodiments, the overall space required toenable such free rotation of the head along the circumferentialdimension can be about 6-15 cm wide, for example 7-12 cm, for exampleabout 10 cm.

In an example, the PET detector heads include detectors that coveroverall about 2-50 cm along the circumferential dimension, for exampleabout 2-40 cm, or 2-35 cm, or 3-30 cm, or 5-28 cm, or 10-28 cm, or 15-25cm, or 20-25 cm, and cover overall about 2-35 cm along the axialdimension, for example about 2-30 cm, or 2-25 cm, or 2.5-20 cm, or2.5-17 cm, or 2.5-15 cm, or 3-10 cm, or 3-9 cm, or 3-8 cm, or 3-5 cm,for example less than 15 cm, and optionally less than 10 cm, or forexample about 5 to 9 cm, or for example 7 to 8 cm.

Dynamically Variable Bore Geometry:

Conventionally, in both PET and SPECT systems, the bore size is notadjustable, and therefore the sensitivity and/or spatial resolution varyaccording to the particular ROI and body location. The conventionalsolution has been to provide a bore adequate to accommodate a full-bodyscan and accept degradation in performance when a smaller bore wouldhave been preferable for a particular ROI as explained below. Anotherconventional solution has been special small-bore systems for brain orneck scans.

According to some embodiments of the present invention, N-M tomographysystems are provided with a dynamically variable-geometry bore. Severalways to achieve this are implemented by providing the degrees of freedomfor gantry configurations and/or detector head extension/retractionand/or angular orientation. In an exemplary embodiment of the invention,these are pre-selectable, i.e., before performance of a scan, and/oradjustable during the scan, either continuously or in steps.

FIG. 4B shows a 12 detector head system with alternating PET and SPECTdetector heads on a circular gantry in which the heads are all in afully retracted configuration providing a maximum bore size for both PETand SPECT operation. Optionally, a non-one to one relationship of thenumber of PET and SPECT is possible.

In the configuration shown in FIG. 5, all the PET detector heads 504have been extended to provide a reduced bore size for PET operation.Alternatively only SPECT detectors are extended. It may be seen thatFIG. 5 represents approximately the maximum extension possible withoutcollision of adjacent heads to the illustrated size and configuration ofthe PET detector heads illustrated.

FIG. 5 also shows exemplary extension/retraction mechanisms 510 and 512respectively for the PET and SPECT detector heads, described more fullybelow.

FIGS. 6A-6D, illustrate spatial resolution improvement that can beachieved when the bore size is increased or decreased to take account ofthe particular ROI and body location. The example shows a SPECT onlysystem with 12 detector heads 602 on one side of a single-ring circulargentry 600, but the same benefits can be achieved in PET only systems,and in systems capable of both SPECT and PET operation.

FIG. 6A shows detector heads 602 in a retracted configuration to providea large bore, for example, a conventional 90 cm. bore, for performing afull body scan. FIG. 6B illustrates detector heads 602 fully extended toprovide a small bore, for example, 20 cm., as it would be used, forexample, when performing a brain scan or a scan of the neck.

It should be appreciated that the cross-section of an ROI will generallynot be circular, particularly in the case of a body scan, so thepossibility non-uniform resolution and/or sensitivity may exist.According to some embodiments of the invention, if desired (or for otherreasons) this can be alleviated in some instances by varying theorientation of the detection surfaces dynamically during the scan. Forthe arrangement illustrated in FIGS. 6A and 6B, this can be achieved byrotating the detector-carrying portions 604 of detector heads 602 aroundan axis parallel to the system axis during a scan. The detectors canoptionally be rotated individually or together. Since such rotation caneffectively fill gaps between adjacent detector heads, it may also allowobtaining good sensitivity and resolution with a smaller number ofdetectors, and thereby result in a less costly system.

It should also be noted that in an exemplary embodiment of theinvention, only the detector element bearing parts of the detector headsneed to be moved. Since these are not heavy, rapid dynamic changes inorientation are practical.

One potential effect (which may be beneficial) resulting from matchingthe bore size to the ROI being examined is an increase in the acceptanceangle for incoming and/or scattered photons. This is illustrated in FIG.6C. Here, a small ROI 604 is assumed to be centered in a bore 606 a thatis large compared to the ROI, or in a bore 606 b that is closely matchedto the size of the ROI. In an exemplary embodiment of the invention, anemission event 605 is assumed to be centered in ROI at 604. As a resultof scattering the two photons travel along angularly displaced paths:607 a for the un-scattered photon and 607 b for the scattered photon,instead of path 607 c. The acceptance angle α, i.e. the angular errorbetween paths 607 b and 607 c, is a function of the bore size and thedetector pixel size according to the relationship:

$\alpha = {2*{\tan^{- 1}\left( \frac{{pixel}\mspace{14mu}{size}}{bore\_ size} \right)}}$

In an exemplary embodiment of the invention, an acceptance angle α1 forthe large bore can be increased to an angle α2 of, for example, up toabout three or four times without degrading or decreasing resolution byreducing the bore size to match the ROI.

Another effect, potentially beneficial which may result from matchingthe bore size to the ROI being examined is illustrated in FIG. 6D. Thisrelates to reduction of the so-called “non-co-linearity effect”resulting from residual momentum of the electron and positronannihilation. A positron event 609 produces emission of a photon pairthat do not travel in exactly opposite directions. One photon travelsalong a path 611 a instead of 611 b and is detected by a detector head608 a instead of 608 b, while the other travels along path 611 c and isdetected by detector head 608 c aligned with detector head 608 b. Bydecreasing the bore size from 90 cm to 20 cm, the photon on path 611 ais detected by detector head 608 b, whereby degradation of resolutiondue to non co-linearity can be reduced by a factor of up to, forexample, 2 or 3 or more with a smaller bore size.

In general, the improvement factor is dependent on the starting andending bore sizes. In the illustration it is reduced from 90 cm to 20cm. The angle error due to non-colinearity is about ±0.25 degree. Thiserror is estimated, for example, based on the energy range of theresidual momentum of the electron and positron annihilation. Theresolution degradation correspond to the shift between the theoreticalevent position (when colinearity is perfect) to the actual eventposition. The error can be estimated with simple trigonometry asErr=tan(alpha)*(Bore_size/2). The error for the 90 cm and 20 cm boresize can therefore be reduced by up to about 4.5.

Adjustment of bore size by extension and/or retraction of the individualdetector heads may be achieved in various ways. A linear motionmechanism can be implemented, for example, with a DC or AC motor, orlinear actuator, by a stepper motor or hydraulically. Positiondetectors, for example, limit switches, resettable counters, or digitalor analog encoders, may be used to provide position feedback and/oravoid over extension and/or collision. The scope of the invention is notnecessarily limited by these methods and other ways will be apparent tothose skilled in the art as well.

FIGS. 6E-6G illustrate an arrangement generally designated at 620 forextending and retracting a detector head 622, according to someexemplary embodiments of the invention. FIG. 6E is a top and sideperspective view, FIG. 6F is a side elevation, and FIG. 6G illustratesan exemplary linear actuator.

Sub-assembly 620 includes a detector head 622 located at one end 624 ofan arm 626, which is movably mounted on a linear rail 628 extendingradially on a gantry 630. Within or (optionally) attached to the outsideof arm 626 is a linear actuator arm 634 (FIG. 6G). Control of extensionand retraction is provided by system controller 30 (see FIGS. 2A-2B).

A weight 632 chosen to balance the weight of detector head 622 and arm626 is moveably mounted on rail 628 and attached to arm 626 for example,by a suitable pulley arrangement and belt or cable or in any othersuitable and/or desired manner. For example, the weight can be between0.5 Kg and 30 Kg, for example, between 3 Kg and 20 Kg, for example, 7Kg. Optionally, the moving part of the detector weighs between 1 and 30Kg. Optionally, the entre arm module weights between 2 and 50 Kg, forexample, between 5 and 30 Kg.

Referring to FIG. 6G, in which some parts are omitted in the interest ofclarity, in the illustrated example, arm 634 includes a driving member636, for example, a sprocket, and a driven member 638 which may also bea sprocket. Input power is provided by a rotary actuator such as a motordescribed above (not shown) attached to sprocket 634 636. A chain (notvisible in FIG. 6G) is carried by sprockets 636 and 638.

Attached to the chain are travelers 642 and 640, which respectivelycarry detector head 622 and its arm 626, and counterweight 632. As willbe appreciated, in an exemplary embodiment of the invention, when thechain is driven, the detector head and counterweight move in oppositedirections, as indicated by arrows 644 and 646.

Such an arrangement allows use of a very small force for extension andretraction while gravitation does not produce any motion or resistancesince the counterweight provides a balancing counter force equal to theprojection of the total force (vector) along the path of linear motion.

In some exemplary embodiments, for a detector head weighting about 20kg, counter-balance can be provided by a weight of approximately 19.5kg, so a force of only about 0.5 kg will be needed to be employed formoving the detector head.

Such a very gentle force potentially reduces risk of patient injury incase of a collision. Moreover, in case of a collision, the patient cantypically easily resist such gentle force and/or move the arms awayregardless of their orientation.

Additional or alternative collision avoidance protection may be providedby proximity sensors of various types. Some options include pressuresensors, acoustic, e.g., ultrasound sensors, and optical sensors mountedon the detector units and coupled to control the actuator motor in asuitable manner In one example, a controller receives an alert when thedistance is below a certain threshold. In another example, the motor isstopped (or a brake activated or motor disengaged) by a dedicatedelectrical circuit reading the sensor.

In some embodiments, the detector heads are brought into close proximityto the patient's body, e.g., within less than 20 cm, or less than 10 cm,or less than 5 cm or within 1-2 cm or in substantial contact or largeror smaller or intermediate distances.

Optionally, in some embodiments, contact with the patient's body isallowed, but the allowable force on the body/skin is limited, forexample, to less than 1000 g, or less than 200 g, or less than 50 g, orless than 10 g or intermediate contact forces. In an exemplaryembodiment of the invention, the contact area is at least 1-10 cmsquared, optionally by having detector not have sharp edges in directionof body.

In an exemplary embodiment of the invention, consideration is given toassuring that adjacent heads do not interfere mechanically oroperationally. FIGS. 7A and 7B, 8A-8D, 9A-9C, 10A and 10B, and 11A-11Cillustrate exemplary design solutions according to some embodiments ofthe invention.

FIGS. 7A and 7B illustrate a dual-mode system 700 comprised of adetector array 706 comprised of six PET detector heads 702 a-702 farranged in three pairs 702 a,d, 702 b,e, and 702 c,f and six SPECTdetectors 704. System 700 is illustrated as being used in PET mode. InFIG. 7A, The PET detectors comprised in array 706 are in and extendedoperating configuration for example suitable for performing full bodyscans. However, because of the shape of the PET detectors, FIG. 7A isalso about the minimum practical extension that is achievable due to thesize and shape of the detector heads.

In an exemplary embodiment of the invention, the controller includes ageometry engine which simulates the space filling properties of thedetectors so as to plan and/or monitor motion in a manner which avoidsinterference between moving (and/or unmoving components). Optionally oralternatively, the geometry engine also calculates lines of sight and/orobstructions. In an alternative embodiment, allowable paths and/orpositions (e.g., for one or more detectors) are pre-calculated andprovided to the system, which uses such paths and/or positions.

To achieve a smaller bore size with detector array 706, only some, forexample one-half of, or any subgroup of, the detector heads, forexample, detector heads 702 b, 702 d, and 702 f, are extended, asillustrated in FIG. 7B. The other detector heads 702 a, 702 c, and 702 eremain in a fully retracted position. In some embodiments, only theextended detector heads are used. In other embodiments, all the detectorheads are used, but the sensitivity and/or resolution of the extendeddetector heads may be not the same as for the un-extended detectorheads. This is optionally accounted for in the course of processing theemission data.

FIGS. 8A-8D illustrate other ways to achieve smaller bores, one suchway, again in a dual-mode system being operated in a PET mode. FIG. 8Ashows the detector element carrying portions of PET detector heads 806a, 806 c, and 806 e rotated 90 degrees around their respectivelongitudinal axes (i.e., in a plane perpendicular to the longitudinalaxes. Compared with FIG. 7A, it may be seen that a smaller bore isachieved.

FIG. 8B shows a configuration in which all the PET detector heads 802have been rotated by 90 degrees, allowing the greatest reduction in boresize.

FIG. 8C represents a situation in which all of detector PET heads 802have been rotated by less than 90 degrees, for example, 45 degrees.

Such rotation may take place, for example before during or afterextension, and/or during a scan as described below.

FIG. 8C illustrates the effect of rotating the PET detector heads 806a-806 f to an angle between 0 degrees and 90 degrees relative to theorientation in FIG. 8A, followed by extension. As may be seen from FIG.8C, such rotation can result in overlap of adjacent detector heads.

In some embodiments of the invention, as the angle of rotation increasestoward 45 degrees, the overlap increases, but decreases as the angle ofrotation increases further toward 90 degrees, at that point, thelimiting case of no overlap is reached, as illustrated in FIG. 8B.

The rotation shown in FIGS. 8A-8D can be implemented in various ways.For example, small bidirectional motors (not shown) may be mounted ongantry 814 and connected through a position tracking arrangement, forexample, using encoders and/or limit switches coupled to detector shafts808 a, etc. Optionally, each detector head unit may include a built-inrotation actuator. Other mechanical arrangements may also be used, forexample, a hydraulic arrangement, or manual adjustment may be also beemployed. Optionally, for this and/or for actuators for extension andretraction, the motors are controlled by the system controller.

Another way of reducing bore size is illustrated in FIGS. 9A-9C. FIG. 9Aillustrates a system 900 in which the individual detector heads 902a-902 f in a detector array 906 are partially extended, for example, fora full-body PET scan, and also are oriented in planes perpendicular tothe respective axes of elongation of the detectors as in the otherembodiments described up to now. However, it should be noted that theextension illustrated in FIG. 9A is about the greatest possibleextension since attempted further extension is blocked by peripheralcontact between the detector heads.

In an exemplary embodiment of the invention, to create clearance forfurther extension and reduction of the bore size, the detector heads aretilted out of the perpendicular planes by pivoting them around axes thatrun parallel (and/or perpendicular and/or other axes that are in theplane of the detector) to the longitudinal axis of the system, e.g., inthe direction parallel to direction of relative movement by which thesuccessive axial slices are produced. As a consequence, as illustratedin FIG. 9B, adjacent detector heads, e.g., heads 904 a and 904 b overlapperipherally allowing a degree of extension of detectors 902 a-902 f notachievable with the heads in the respective perpendicular planes asillustrated in FIG. 9A.

Optionally, to achieve yet further reduction in bore size, heads 904a-904 f are tilted even further as illustrated in FIG. 9C therebyincreasing the overlap and allowing additional extension.

The tilting shown in FIGS. 9A-9C can be implemented in various wayssimilar to those employed in the embodiments of FIGS. 8A-8C, as will beunderstood by persons skilled in the art.

It is noted in these and other embodiments, that the detector plane neednot be flat. For example, it may be curved being a section of acylinder, a sphere and/or other conic section and/or other curved and/orpiecewise linear shape.

FIGS. 10A and 10B illustrate a detector array 1000 employing a furtherway of achieving a wide range of bore sizes, which is optionally usedtogether with other methods as described herein. Here, detector array1000 includes six PET detector heads 1002 a-1002 f and six SPECTdetector heads 1004 a-1004 f mounted on a gantry ring 1006. Toaccommodate the shape of the PET detectors, detector head pairs 1002 a,1002 d and 1002 c, 1002 f are located in a first plane on the gantry,while intervening detector head pairs 1002 b, 1002 e, are located on asecond plane spaced axially from the plane of the other two pairs.

FIG. 10A illustrates extension of the PET detectors to achieve a firstdesired bore size. To achieve a smaller bore size, the detectors areextended as desired, as shown in FIG. 10B. Because of the axial spacing,even after the detectors have been extended sufficiently that they wouldcome in peripheral contact if they were situated in a single plane,further extension is possible because the axial spacing allows the tipof one detector, for example 1002 a, to pass behind the adjacent tip ofthe next detector 1002 b.

As will be appreciated, suitable modification of the programming of thecoincidence detection sub-system may be made to account for the axialspacing. In some embodiments the programming does not need to bemodified (but only parameters or look up tables) since it just needs tonote the non-uniform viewing when determining relativecounts/normalizing from different areas (e.g., using sensitivity mapsand/or other calibration maps).

FIGS. 11A-11C illustrate another arrangement for achieving a wide rangeof bore sizes involving axial spacing, which may be used, for example,alone or with other methods described herein. FIG. 11A is an end view ofassembled detector array 1100. FIG. 11B is a perspective view with thetop portion cut away and FIG. 11C is a perspective view rotated 90degrees from that of FIG. 11B to show internal construction details.

Here, detector array 1100 is comprised of six PET detectors 1102 and sixSPECT detector heads 1104. To accommodate the shape of the PETdetectors, detector pairs 1002 a, 1002 d and 1002 c, 1002 f are locatedon a first ring 1106 of a gantry 1108, while intervening detector pair1102 b, 1102 e, is located on a second ring 1110 spaced axially from thering of the other two pairs. Optionally, since ring 1110 carries onlyone PET detector pair, all the SPECT detectors may be mounted on thatring.

Optionally, more than two rings may be provided. FIG. 11D shows anembodiment in which the detector heads are arranged in three layers,either on one ring or on three separate rings. Additional rings, forexample, 3, 4, 5, 6 or more may be provided (and optionally added in amodular manner). In an exemplary embodiment of the invention, thedetectors on different rings are of different types, sizes and/orqualities.

Detector Head Movement on the Gantry and Gantry Rotation:

A potential benefit of the variable geometry aspect of some embodimentsof the invention is the possibility of obtaining good resolution andsensitivity around the entire ROI with a reduced number of detectorunits that are laterally or circumferentially moveable on a gantry. FIG.12A illustrates schematically one way of translating detector heads on agantry. Here, eight detector heads 1202 a-1202 h are slidably mounted ona track assembly 1204 comprised of separated track segments 1206 a-1206h.

In an exemplary embodiment of the invention, heads 1202 may be movedalong their respective track segments by a linear motion arrangementsimilar to that described in connection with FIG. 6G, or by any othersuitable and desired arrangement.

Various movement options may be provided, as will be apparent to thoseskilled in the art in light of the disclosure herein, for example, butnot limited to (a) prepositioning (b) steps of a step and shoot regimen,(c) adjustment of position between axial slices, and (d) “on the fly”adjustment during gantry rotation or a spiral scan. It should berecognized that combination of the indicted or other options are alsocontemplated. Further it should be recognized that the detector headsmay be positioned either uniformly or non-uniformly around the gantry.Movement over any desired range depending on the number of detectorheads possible, for example, 20 degrees (i.e., ±10 degrees from acentral position). The described arrangements are applicable to PET andSPECT detector heads as well as dual purpose heads as described below.

FIG. 12B illustrates an alternative arrangement in which the detectorhead move circumferentially on the gantry. Here, a detector array 1208mounted on a gantry 1209 includes six detector heads 1210 a-1210 f. Withrespect to at least some designs, the concepts being described areequally applicable to PET and SPECT detectors. As in other embodimentsdescribed, the detector heads can be extended and/or retracted, and/oroptionally rotated or tilted, to vary the bore size.

In an exemplary embodiment of the invention, for example, to achieve avariation in spatial resolution around the ROI, some of the detectorheads, for example detector heads 1210 a, 1210 c, and 12104, aremoveable circumferentially on gantry 1209 as indicated by arrows 1212 a,1206 c, and 1212 e. As a result, in the embodiments of both FIGS. 12Aand 12B, there can be more detectors in some areas, and in other areas,there can be fewer detectors. Optionally, detector heads can beconcentrated in the wide areas to provide enhanced resolution in thoseareas. Optionally or alternatively, different detector head qualitiesare used in different areas, for example, to support non-uniform datacollection protocols.

A possible benefit of the translational embodiment of FIG. 12A is thatit may be easier to implement. Arcuate motion, or translation over arange of ±about 15 to 30 degrees, for example, ±20 degrees from acentral position can give good results.

Another way to enhance resolution around an ROI with a reduced number ofdetector heads is to provide gantry rotation for PET operation as inSPECT operation. This concept is illustrated in FIG. 13. In anon-rotating system, reducing the bore size from the conventional 90 cmto 30 cm for a small ROI for a particularly configured detector headresults in a larger angle of acceptance α2 as compared to α1, but maydecrease the angular resolution, e.g., as previously noted. However, ifthe gantry is continuously rotated, or continuously rotated duringimaging of successive axial slices, e.g., with the same number ofdetector heads, and a bore size of 30 cm, the angle of acceptance α3 canbe made smaller, for example, smaller than even α1, and the lost angularresolution can be recovered. Optionally, the speed of rotation isselected according to the desired acceptance angle. In an exemplaryembodiment of the invention, with a rotating gantry, the detector arraycan be configured with a number of detector heads arranged over lessthan the full 360 degrees around the ROI, for example, over between 180and 320 degrees. In a static system, that would result in a potentiallylarge gap in coverage. Rotation assures that emission events from allparts of the ROI will be detected as the detector array rotates, eventhough generation of the image data may require more time.

A rotating gantry can also be provided in embodiments in which thedetector heads are mounted on axially spaced gantry rings and/ortranslatable on the gantry, yielding the benefits of both a wide rangeof bore configuration adjustability and increased angular resolutionwith fewer detector heads. Optionally, the two gantry rings can bearranged to rotate at different speeds.

FIG. 14 illustrates a potential benefit of varying the detector geometryduring a full-body scan. For a normal patient 1402, a small bore is usedfor the slices in the regions 1402 a and 1402 b of the lower legs, neck,and head, and a larger bore size for the region 1404 c of the upper legsand the torso. In contrast, for an obese patient 1404, a larger number,for example, four bore sizes gives better results. Thus, for regions1404 a and 1404 e covering the lower legs, the head and the neck, afirst bore size is used. For regions 1404 b and 1404 c covering theupper legs and the upper torso, a second larger bore size is used. Forregion 1404 d covering the lower torso and the chest, a third bore size,even larger than those for the other regions is used.

A further option is to vary the bore size on-the-fly as the scanproceeds, resulting in a bore size that dynamically follows the contourof the patient's body.

It should also be noted that for an ROI that is relatively small,decreasing the bore size can be facilitated by making the patientcarrier transversely adjustable or with a part that is narrower than theoverall width.

Exemplary Method of Use:

While it is believed that the method of use of the various embodimentsdescribed above should be apparent to those skilled in the art from theforegoing description, this may be summarized in conjunction with theflow diagram of FIGS. 15 and 16. For purposes of discussion, it isassumed that a PET procedure or a SPECT procedure is to be performedeither in a single or dual mode system, but it should be understood thatthe discussion is also applicable to simultaneous performance of PET andSPECT procedures.

As shown, at 1502, suitable preparation of the patient, includinginjection of the radioactive tracer is optionally undertaken. At 1504,optionally while (and/or before, and/or after) the tracer is circulatingthrough the bloodstream to the ROI, the size and shape of the ROI isdetermined, for example by a conventional transmission CT. Optionally,the CT may be performed using a CT capability (e.g., using an x-ray orradiation source) included in the N-M tomography system itself, or byuse of a separate CT system.

At 1506, the bore geometry and if necessary, the configuration of thecollimator septa are adjusted to accommodate the size, shape, andlocation of the ROI and the desired spatial resolution according to thenature of the procedure being performed. Depending on the required sizeof the bore, the detectors are extended as needed. If the detectorscannot be extended sufficiently to provide as small enough bore, onlysome of the detectors are extended. Alternatively, according to thefeatures of the particular system, the detectors are rotated and/ortilted to the required angular orientation, and optionally then thedetectors are extended.

At 1508, after sufficient time (and/or during this time) for the tracerto travel through the patient's bloodstream to the ROI, the scan isperformed, and at 1510, the image reconstruction is performed.

FIG. 16 illustrates a more complex scan procedure, again applicable ingeneral to a PET or a SPECT procedure, or simultaneous performance ofboth PET and SPECT procedures. Here, 1602 and 1604 are the same as 1502and 1504, but at 1606, a scan regimen is programmed for in-scan detectorand collimator geometry variation. This may include one or more of thefollowing features:

-   -   a) Preliminary bore size adjustment;    -   b) Continuous gantry rotation during the scan or at each axial        slice;    -   c) Gantry rotation speed adjustment;    -   d) Continuous bore size adjustment (both extension and angular        orientation of the detector heads) during the scan at particular        axial positions, or continuously over the entire scan;    -   e) Variable positioning of the detector heads on the gantry        (initially or over the course of the scan);    -   f) Variable collimator configuration initially and/or over the        course of the scan.

At 1608, after sufficient time for the tracer to travel through thepatient's bloodstream to the ROI, the scan is performed according to theprogrammed regimen, and at 1610, the image reconstruction is performed.

Detector Unit Arrangements and Configurations:

Referring again to FIGS. 2A-2B, it will be recalled that a conventionalSPECT detector unit is comprised of an emission detector element 32 inthe form of a scintillator that provides an optical (or electrical)light pulse in response to impingement of a gamma-ray photon, an arrayof photomultiplier tubes (PMT) 34 that convert the optical light pulsesinto electrical signals from which the images are reconstructed and acollimator arrangement 42 with openings aligned with the PMTs to providea narrow acceptance angle for each PMT, i.e., to ensure that photonsstriking the detectors do so at a relatively narrow range of angles.Alternatively, the scintillators and the PMT can be replaced by a directconversion semiconductor detector, for example, a SiPM as describedherein.

Conventional PET detector units can be similarly configured, but do notinclude a collimator array because incidence angle information isextracted based on coincident detection of two photons emitted by asingle radioactive decay.

Scintillation detector elements are typically formed as unitarystructures, and such structures are also employed in some embodiments ofthe invention. Alternatively, the detector elements according to someembodiments are pixilated, e.g., formed of an array of discrete smalldetector pixels. This can be advantageous particularly for PET imaging,in that it enables identifying the orientation from which the photon hasbeen emitted at a much finer precision, for example, along a line with awidth of about 4-6 mm, between two locations of coincident detectionstaking into account the distance of positron travel before annihilationand the pixel width.

In an exemplary embodiment of the invention, dual use (PET and SPECT)detector systems are provided.

In an exemplary embodiment of the invention, pixilation may in somecases facilitate optionally providing time-of-flight analysis circuitryfor PET operation to determine where along the estimated emission linethe positron was emitted, for example at a longitudinal resolution of,for example, about 1-5 cm, for example 2-3 cm (e.g., by measuring suchtime per pixel or group of pixels).

Optionally, the electronic circuitry connected to some or all of thedetector units can also provide photon energy characterization (e.g.energy level) within the entire SPECT and PET range of about 40 KeV to511 KeV, and detection of count rate in case of high flux of photons,for example when a high intensity radiation source is activated such asX-ray source, and/or detection time resolution sufficient forcoincidence and time of flight detection.

Optionally, the signal processing electronics can also include multipleseparate channels that allow independent amplification and front-endprocessing for each detector or small group of detectors and/or a smallnumber of pixels (e.g., between 10 and 1000, for example 100), such thatany malfunction of one or more pixels or detectors and any blinding ofone or more pixels or detectors by a “hot spot” (high intensity source)do not prevent other detectors from properly functioning and detectphotons emitted from other regions. One suitable way to achieve this isshown in commonly assigned U.S. Pat. No. 8,445,851 commonly ownedherewith, the content of which is hereby incorporated herein in itsentirety as if fully set forth. The detector pixels can be arranged invarious configurations according to embodiments of the invention. In anexample, the detector can be configured as a symmetrical matrix of 8×8pixels, 10×10 pixels, 12×12 pixels, 16×16 pixels 20×20 pixels, 32×32pixels, or larger or smaller or intermediate sized matrices.Alternatively, the pixels may be arranged in asymmetric configurations,for example 16×32 pixels, 16×64 pixels, 8×16 pixels, and other larger,smaller or intermediate sized configurations.

In a non-limiting example, detector pixels have dimensions in the rangeof about 0.1-20 mm, for example, about 0.2-15 mm, for example 0.5-10 mm,for example 1-5 mm, for example 1-2 mm or 2-3 mm or 2-4 mm.

While in an exemplary embodiment of the invention the pixels aresymmetric (e.g., square), this need not be the case, for example, thepixels may be elongate in a certain direction, for example, having afactor of between 1.1 and 4 or more between two orthogonal dimensionsthereof.

In another non-limiting example, the pixel pitch (i.e., spacing betweenpixels), is symmetrical in two directions, for example, about 2.5 mm orabout 1.25 mm or about 1 mm, or about 2 mm or about 3 mm, or larger, orsmaller or intermediate values. In another example, the pixel dimensionin one direction is different than the dimension in another direction,for example 2×3 mm, 1.5×2.5 mm, 2×2.5 mm, etc.

In some exemplary embodiments, the detectors have dimensions in therange of about lcm to about 15 cm, for example in the range of 2 to 8cm, for example in the range of 3 to 5 cm, for example 4 cm.

Exemplary Reconstruction Variations

In some exemplary embodiments of the present invention photons aredetected by solid state detectors and electronic circuitry that areconfigured for acquisition of single photons of typical SPECT energylevels, and/or single photons of high energy such as 511 KeV, and/orcoincidence detection of pairs of photon received as a result of asingle positron emission from the radiopharmaceutical. In exemplaryembodiments of the invention detector modules are capable of detectingmore than one of these modes, for example detecting a wide range ofenergies single photons such as from 40 KeV to 511 KeV. In otherexemplary embodiments of the invention detector modules are capable ofdetecting 511 KeV photons both as single events (if coincidence of apair of photons was not detected) and as coincidence photons detection.

In an exemplary embodiment of the present invention a collimator is usedon the detector which provides wide collection angle of 511 KeV, butwith some preferred orientation of detection (for example, about 20%, or30%, or 50%, or 70%, or 100% or 150% or 200% or 300% higher probabilityin a certain direction (e.g., having an angular aperture of between0.001 and 10 degrees in a largest dimension) compared with most otherdirections, for example about 100% higher, which is about twice theprobability, in a main direction).

In an example, information from photons that are detected as part of acoincidence is processed by the reconstruction algorithm as beingprobably received from a location along a line of sight between the twodetection locations, and information from photons that were detectedonly in one location with no detection of the coincidence can be eitherignored or being processed in a SPECT-like probability analysis based onthe detection probability function (“functional”, detection probabilitymap) which depends on the collimator properties and its preferredorientation.

In an example, the reconstruction algorithm combines information fromboth SPECT-like analysis and coincidence-like analysis. In an example,the analysis based on individually detected photons (as a singledetected photon, and/or as each of a pair of coincidence photons) isused to form an interim information, for example interim reconstructionof the radiopharmaceutical distribution in 3D volume, and that interiminformation is used as a prior information to a further 3D imagereconstruction based on coincidence analysis. In this approach, forexample, analysis based on one approach is either fully integrated with,and/or iteratively integrated with, and/or used as a prior info for,analysis based on the other approach. In an example, PET-like analysisserves as prior knowledge for SPECT-like analysis. In an example,SPECT-like analysis serves as prior knowledge for PET-like analysis. Inan example, the SPECT-like analysis and the PET-like analysis areiteratively performed, either one providing for preprocessing or priorknowledge for the other, or one serves for post-filtering of the other,or algorithms merged with one-another, or any combination thereof.

In an exemplary embodiment of the invention, when reconstructing datafrom multiple energies, a priori probability of correlation between twoenergies is optionally used. For example, for a given body structure,the apriori probability of a SPECT event (or event at one energy) may bedifferent from the a priori probability of a SPECT or PET event at adifferent part of the structure and/or at the same part. Optionally, aprevious image is used as an input to indicate the differences inradiopharmaceutical distributions at the different detected energies. Inone example, heart muscle is detected using one energy and a diseasedlocation in the heart is detected using another energy. Reconstructionof the shape and/or location of the heart using the first energy may beused to limit (e.g., anchor) the other energy to fit within theboundaries of the heart as reconstructed by the first energy or asmatching a model sized and shaped using the first energy.

In an exemplary embodiment of the present invention the reconstructionalgorithm is adaptive to take into account variable location of thedetectors. Unlike conventional PET, where the algorithm assumes thatdetectors positions and orientation is known and fixed in advance, andin particular the relative location and orientation of the detectors isknown (one detectors relative to the other detectors), in an example ofthe present invention the algorithm forms probability distribution mapsthat factor the de-facto location of the detectors during theacquisition, as customized per patient and/or instant of time.

In an example, the distance between detectors and the body, the locationof the detectors in space and the relative position of the detectors(one relative to others) varies from one detector to another and fromone patient to another. Moreover, in an example, the algorithm isconfigured differently than conventional PET-reconstruction algorithmsto form probability maps for photons to be detected by a detector, to becalculated based on the location and orientation of the detector duringthe acquisition. The probability to obtain coincidence detection changesas the detectors move and are positioned closer to the body, as the lineof sight between any two detectors becomes different than that which waspre-fixed in conventional ring-based detectors.

In an example, a 3D image reconstruction algorithm calculates aprobability function (“the functional”) of a radiopharmaceutical for avoxel (a small volume in a certain 3D location in space) to emit apositron that converts to 2 photons (following annihilation of thepositron) that would be detected as a coincidence event by a pair ofpixels (one from each of two opposing detectors), taking into accountthe position of the detectors and their orientation as a result of thedetector motion in-out towards the body. In an example, the probabilityof detection as a single photon event is calculated too, taking intoaccount, for example, the position of the detectors as a result of thedetector motion in-out towards the body and/or collimator or detectordesign. In an example, the orientation of the detectors is also beingused as part of the calculation of the probability function.

In an example, the detectors also move laterally, for example by motionof the gantry and/or linear motion and/or rotation thereof, and thereconstruction algorithm forms the probability functions taking intoaccount the gantry lateral motion and its effect on the position andorientation of the detectors. In some examples, such motion is donebefore photon acquisition begins, and the algorithm accounts for it. Inanother example, such motion occurs during photon acquisition process,and the algorithm accounts for it by having the probability functioncalculated to take into account the dynamic changes due to the relativemotion during the scan.

In an example, the detectors rotate around one or more local axes, forexample by rotating around an axis of rotation per detector structure(or per group of detectors), for example, independently rotatingdetectors (or groups) around an axis which is more-or-less parallel tothe main axis of the patient body. In this example, the reconstructionalgorithm forms the probability functions taking into account thedetector rotation. In some examples, such rotation is done before photonacquisition begins, and the algorithm accounts for it. In anotherexample, such rotation occurs during photon acquisition process, and thealgorithm accounts for it by having the probability function calculatedto take into account the dynamic changes due to the orientation changesduring the scan.

In an example, a combination of the some or all of above components ofthe reconstruction algorithm is used to enable adaptation of thereconstruction algorithm and use of the probability functions takinginto account the ability of the detectors to move before and/or duringthe scan. For example, the adaptation is provided using a sensitivity orenergy correction and/or by modifying the model of the detectors used inreconstruction. As noted herein, in some cases planning or RTacquisition is modified, for example, to ensure a sufficient photoncount from a desired location, to ensure a desired quality, to avoiddata collection from undesired regions and/or to ensure stability ofreconstruction.

In another example of the present invention the system is capable ofsimultaneously acquire high energy (e.g. PET, 511 KeV) and lower energy(SPECT, X-Ray) photons. In an example, such simultaneous acquisition ofphotons from multiple energy levels allows simultaneous imageacquisition of multiple radiopharmaceuticals. For example, simultaneousimaging of radiation from PET isotope (e.g. radiopharmaceutical based onone or more of F-18, C-11, N-13, O-15, Rb-82, Cu-62, Ga-68, Iodine), andfrom a SPECT isotope (e.g. radiopharmaceutical based on one or more ofTc^(99m), Tl²⁰¹, I¹²³, In¹¹¹). In an example, the acquisition of photonsemitted by two or more radiopharmaceuticals is simultaneous, and theimage reconstruction algorithm generates 3D images of the distributionof two or more radiopharmaceuticals within the ROI, thus avoids problemsassociated with registration of images from different sources that areacquired by different systems and/or at different time.

While the term PET has been used for convenience, other coincidencedetection methods may be used. Similarly, other single photon detectionmethods than SPECT may be used.

Such a system as described herein may be selectively operated, forexample, in a single mode (coincidence or non-coincidence) and/or in adual mode.

In an exemplary embodiment of the invention, for coincidence detection,time stamps per photon are obtained at about microseconds orsub-microsecond resolution, and time-of-flight processing is optionallyobtained if time-stamp is obtained at sub-nanosecond resolution. Inthese cases, a processing means, such as a central CPU (in this or otherembodiments) can identify the matching photons and analyze thecoincidence emission line, and if available also the estimated positionalong the line based on time-of-flight calculation.

Collimator Arrangements and Configurations:

FIGS. 17-22D illustrate exemplary collimator configurations, including,for example designs that are adjustable to provide high and low spatialresolution. This capability may be advantageous for SPECT imaging usingN-M tomography systems providing adjustable bore geometry as discussedbelow, and also to permit selectably using the same detector units forPET or SPECT imaging, or for both PET and SPECT imaging simultaneously.In some embodiments, no physical adjustment is applied, for example,using software correction to adapt the received signal for differentdesired collimation conditions.

Referring first to FIG. 17, there is shown a representation of anexemplary basic adjustable collimator designated 1700. For referencepurposes, the Z-direction is taken as perpendicular to the plane of thesensor element and the Y-direction is the direction parallel to themachine or patient axis, The X-direction is orthogonal to the Y and Zaxes, and with the Y-axis defines a plane parallel to the plane of thesensor element. It is note that non-rectangular collimators may be used,for example, hexagonal.

As illustrated in FIG. 17, collimator 1700, is formed of two orthogonalsets of septa 1702 and 1704. Septa 1702 lie in X-Z planes (spaced in theY-direction). Septa 1704 lie in Y-Z planes (spaced in the X-direction).Septa 1702 are formed with slits 1706 at spaced intervals in the Xdirection. Septa 1704 are formed with slits 1708 at spaced intervals inthe Y direction. The spacing between slits 1706 corresponds to theX-direction spacing (or multiples thereof) of septa 1704, while thespacing between slits 1708 corresponds to the X-direction spacing (ormultiples thereof) of septa 1702.

Septa 1702 and 1704 fit together as shown to form an “egg-crate” arrayof collimator cells 1710 that can easily be assembled. As illustrated,the spacing of septa 1702 (in the Y direction) and 1704 (in the Xdirection) are the same so that cells 1710 are square. However, itshould be understood that the septa spacing can be different in the Xand Y directions so that cells 1710 are rectangular. Another variationis to make the slit spacing and/or the spacing of septa 1704 non-uniformallowing different size cells at different locations in the collimator.These variations are simple to achieve with the illustrated design. Aplastic frame (not shown, optionally positioned between the collimatorand the ROI) is optionally used to press the septa together and againstthe detector, for example, using one or more screws to provide thepressure. Other attachment mechanism may be used as well, for exampleconnecters which interconnect the speta and/or attach a flange at theend of one or more septa to the detector.

FIG. 18A-22D illustrate exemplary (but non-limiting) ways to vary theresolution of collimators according to some embodiments of theinvention. Before proceeding to a description of some of theseembodiments, the following points should be noted:

1. FIG. 18A shows a fragment 1800 of a collimator formed by spaced Y-Zplane septa 1802 and X-Z plane septa 1804. These define collimator cells1806 (four of which are shown) that provide photon travel paths to thedetector element. For simplicity, FIGS. 18A-22D show only part of asingle row of the collimator. The full collimator generally includesmultiple (e.g., 4, 10, 20, 30 or intermediate or greater numbers)parallel rows like 1800. Also, while only four cells 1806 are shown, theactual number of cells (as well as the number of parallel rows 1800)will depend on the overall dimensions of the emission detector element,and the pitch of the septa (e.g., the spacing between septa or septacenters).

2. In some embodiments, the septa pitch in the Y-direction is the sameas that in the X-direction whereby the individual collimator cells 1806are square. Alternatively, the septa pitch in the Y-direction isdifferent from that in the X-direction whereby the individual collimatorcells 1806 are rectangular. Alternatively or additionally the pitch canbe variable in the X and/or Y directions. In some embodiments the septaare not straight. For example, the arrangement may be of radially (e.g.,extending from one or more points) arranged fixed septa and circularspeta mounted thereon. Optionally or alternatively, one or more septamay be inclined away from a perpendicular to the detector.

3. In some embodiments, in addition to or instead of different pitch inthe X and/or Y directions, and/or variable pitch in the X and Ydirections, the septa thickness may be different in the X and/or Ydirections. As an additional or alternative option, the septa thicknessmay be variable in one or both the X and Y directions. As anotheralternative or additional option, the septa length may be different inthe X and Y directions. As yet another alternative or additional option,the septa length may be variable in one or both the X and Y directions.The variation within a detector collimator for one or more of theseparameters may be, for example, within a factor of 1.1, 2, 3, 4, orintermediate or greater factors. Thus, some embodiments providenon-uniformly sized and/or shaped collimator cells.

4. Collimator configurations and geometry variations as illustrated inFIGS. 18A-22D can be used with both pixilated and with non-pixilateddetectors.

5. Conventional PET detectors do not include collimators. However, ithas been found that certain materials, for example, tungsten and theothers mentioned above more efficiently absorb photons having energy inthe 40-250 KeV range emitted by tracers used for SPECT imaging than thehigh energy (511 KeV) PET photons. Forming the collimator septa of suchmaterials helps make it possible to use detectors having collimators asdescribed herein for both PET and SPECT imaging with only slightly lessefficiency (i.e., sensitivity, e.g., 30%, 20%, 10% or intermediate orsmaller reduction of sensitivity) in the PET mode, but with widereffective viewing angle (i.e., angle of acceptance) since the highenergy photons are able to pass multiple septa, for example, 3, 4, 5 or6 septa. Stated differently, despite the decrease in detectionprobability (resulting from decreased sensitivity as the angle ofacceptance increases), useful SPECT detectors can still provide higheffective sensitivity for PET detection.

6. In exemplary embodiments, blockage for PET energies is less than 80%,60%, 50%, 20% or intermediate percentages depending, for example, on theacceptance angle, septa design, and/or material as described herein. Asa specific non-limiting example, the collimator may be formed oftungsten septa about 0.2 mm in thickness, and having 1.03 mm squarecells, with a pitch of 1.23 mm and height in the Z-direction of 14 5 mm,and overall horizontal septa length of 10.8 mm

7. Use of wide angle reconstruction techniques for SPECT imaging, forexample, the ML-EM algorithm, allows a single adjustable-geometrycollimator to be used for SPECT imaging for multiple distances betweenthe detector and the target area, e.g., a range factor of 1.5, 2, 3, 5or intermediate or greater values. Suitable known reconstructiontechniques for PET imaging include without limitation, FBP (filteredback projections), iterative algorithms with or without PSF (pointspread function), and modeling. These may also permit collimateddetector units formed for example, of tungsten septa to be used for PETimaging to detect photons over a range of detector angular extent alongthe system axis, for example, e.g., a range factor of 1.5, 2, 3, 5 orintermediate or greater factors.

8. It should further be noted that conventional SPECT algorithms assumea collection angle and a detection probability map (resulting from thecollimation and perhaps other factors) that is part of the algorithm. InPET imaging, as the detectors are exposed to the entire imaged volume,the coincidence line is used for the reconstruction process. However, inthe some of the exemplary embodiments, the variable septa geometryaffects PET reconstruction to some extent, in steps—sensitivity beingchanged (and generally reduced with some steps) as the detection angleincreases. In an exemplary embodiment of the invention, the algorithmssuitable for PET image reconstruction advantageously take account of thesepta configuration to attribute to photons coming from a particularcoincidence line a detection probability, such that the probability isfactored into the calculation: lower probability means that the sourcein that direction is actually “hotter”/“brighter” than it looks.Therefore, the value attributed to radiation received by a particularpixel is related to the counts received from that direction divided bythe probability of detection from that location. This concept is validfor both SPECT and PET reconstruction because the probability ofdetection varies in different angles related to the existence of thecollimator and the septa configuration, but also the fact that differentangles of detection hit different effective thickness of septa, the factthat collimation may be variable, and/or the variable geometry of thedetector heads as previously described.

FIGS. 18A-18C illustrate an embodiment in which spatial resolution isenhanced by lengthening the Y-Z plane septa 1802 and/or the X-Z septa1804 thereby narrowing the angle of photon acceptance.

In FIG. 18A, collimator fragment 1800 is shown in an un-elongatedconfiguration which provides a wide angle of acceptance for SPECTimaging. for example, ranging from about 1 and about 30 degrees, andadvantageously, between about 5 and about 15 degrees FIG. 18B showselongation of the collimator in the Z-direction to reduce the angle ofacceptance and thereby improve the spatial resolution. FIG. 18B showsextension of only the X-Z plane septa 1804, while FIG. 18C showselongation of both the X-Z septa and the Y-Z septa. It should beunderstood that alternatively for the situation illustrated in FIG. 18B,the Y-Z plane septa 1802 can be extended instead of X-Z plane septa1804.

Extension and retraction of septa 1802 and 1804 may be achieved by anysuitable and desired mechanism. This is shown schematically as acoupling rod 1806 for septa 1804, A small motor, for example a steppermotor or a multiple position relay (not shown) attached to coupling rod1808 provides for step-wise movement. Alternatively, continuousadjustment can be provided, for example, by a suitable motor andposition sensors. The septa can also be extended and retracted manually.

Extending either septa 1802 or 1804 as described can be useful, forexample to increase resolution in a certain orientation—while possiblyreducing sensitivity in that direction. Retracting the septa does theopposite. In an exemplary embodiment of the invention, tilting of speta(e.g., by moving a bore-side thereof while using the detector side as apivot) is used for when the detector is rotated out of plane duringarrangements as described above. A louver-like mechanism may be used.

In an exemplary embodiment of the invention, the reconstructionalgorithm is adapted for the collimator configuration, for example,based on a look-up table with different parameters for differentcollimator configurations. Optionally, a sensor which reports thecollimator configuration or the command for collimator adjustment isused to calculate such parameters and/or as input to a reconstructionalgorithm Which uses such measurement to modify the reconstruction(e.g., sensitivity correction and desired count rate for stability ofreconstruction and/or image quality).

In an exemplary embodiment of the invention, planning of dataacquisition takes into account possible collimator variations, forexample, by calculating desired configurations and/or by comparingoptions with different collimator arrangements.

In an exemplary embodiment of the invention, when acquisition depends onpervious acquisition, collimator and/or detector configuration arechanged in a manner which preferentially provides data (photondetections) from a desired location and/or time and/or to preferentiallyblock data from a certain location and/or time.

FIGS. 19A and 19B illustrate an embodiment in which the pitch of thesepta is varied. FIG. 19A shows a collimator fragment 1900 comprised ofX-Z plane septa 1902 and Y-Z plane septa 1904 defining collimator cells1906. FIG. 19A shows a configuration in which the

X and Y dimensions of the collimator elements are equal. FIG. 19B showsX-Z plane septa 1902 shifted in the Y-direction by one-half the septapitch. Alternatively, Y-Z plane septa 1942 may be shifted (in theX-direction). In either case, the cross-sectional area of the opening incollimator cells 1906 is reduced by for example, 25-75 percent, forexample, 50 percent thereby decreasing the acceptance angle andincreasing the spatial resolution.

In some embodiments, both (some or all thereof) the X-Z and Y-Z planesepta 1902 and 1904 can be shifted, thereby reducing the area of thecollimator cell openings, for example, by 75 percent, and furtherdecreasing the acceptance angle and correspondingly increasing thespatial resolution.

Moving septa 1902 and/or 1904 can be effected using an arrangementsimilar to that described in connection with FIGS. 18A-18C (but adaptedto provide Y-direction movement) or in any other suitable and desiredmanner

FIGS. 20A and 20B illustrate another embodiment in which the pitch ofthe septa is varied. In FIG. 20A, a collimator fragment 2000, is formedby X-Z plane septa 2002 and Y-Z plane septa 2004 defining collimatorcells 1906. As in FIG. 19A the septa pitch is the same for septa 2002and 2004.

FIGS. 20A-20B show the upper ends of X-Z plane septa 2002 tilted in theY-direction by one-half the septa pitch. Alternatively, Y-Z plane septa1942 may be shifted (in the X-direction). In either case, the area ofopening in collimator elements 1906 is reduced by, for example, by 25-75percent, for example, 50 percent thereby decreasing the acceptance angleand increasing the spatial resolution.

The tilting may be achieved in generally the same manner as in theembodiments of FIGS. 18A-18C and 19A-19B, except that the bottom ends2008 of septa 2002 are pivotally mounted. In some embodiments tiltingprovides a parallel collimation. In other embodiments, it provides afan-in collimation or a fan-out collimation. Different adjustment andadjustment types may be provided for different detectors and/or fordifferent parts of a same detector, for example, during a same sessionor even simultaneously, for example, responsive to ROI location, ROItype, time form injection of tracer and/or arm and/or bore geometry.

FIGS. 21A-21D illustrate resolution adjustment using an arrangement oflayered or vertically tandem collimator sub-units or parts, inaccordance with some embodiments of the invention. FIGS. 21A and 21B arerespectively an end elevation and a side perspective view of a two partcollimator fragment 2100 comprised of first sub-unit 2102 and a secondsub-unit 2104 located above sub-unit 2102. In this context, the word“above” is to be understood as meaning closer to the detector element.Therefore, sub-unit 2102 is positioned closer to the source of radiationthan sub-unit 2104.

Sub-unit 2102 is formed a first set of septa 2106 extending in an X-Zplane, and a second set of septa 2108 extending in a Y-Z plane. Sub-unit2104, in contrast, is formed only of one set of septa 2110 extending inthe X-Z plane. Alternatively, sub-unit 2104 can be formed of two sets ofsepta like sub-unit 2102.

Also, while sub-unit 2104 is above sub-unit 2102, alternatively, thesub-units can be reversed so that sub-unit 2102 is above sub-unit 2104.

FIG. 21B shows two ways the collimator cells of collimator 2100 may bedecreased in size to provide a smaller acceptance angle, and thereforehigher resolution for SPECT imaging: septa 2108 can be moved in theX-direction and septa 2110 can be moved in the Y-direction. If onlysepta 2108 are moved, the area of the cell openings is decreased by upto 50 percent. If both sets of septa 2108 and 2110 are moved, cellopening area can be decreased by up to an additional 25 percent.

FIG. 21C illustrates an alternative collimator configuration 2112comprised of an upper sub-unit 2114 formed by X-Z plane septa 2118 and alower sub-unit 2116 formed by X-Z plane septa 2120 and Y-Z plane septa2122. In this embodiment, sub-unit 2114 includes a second set of X-Zplane septa 2124 positioned between septa 2110, and sub-unit 2102includes a second set of Y-Z plane septa, one of which is shown at 2126positioned between septa 2122. In this configuration, only intermediatesepta 2114 and 2126 are moveable.

FIG. 21D shows a further alternative embodiment 2128 in which the septaare configured as in the embodiment of FIG. 21B except that the septa2122 and 2130 are tiltable rather than slidable in the Y and Xdirections, respectively.

An un-illustrated variation of collimator 2112 of FIG. 21C has threesub-units or parts in vertically tandem relationship. The three partsmay be of equal length in the Z-direction, or any other desiredproportion, for example, 1:2:1 (i.e., so that middle part is twice aslong as the top and bottom parts and thereby provides one-half thelength of the collimator. Other proportions are also possible.

Optionally, collimator resolution in this embodiment is increased bymoving the Y-Z plane septa forming the middle part in the X and/or Ydirections as in the embodiment of FIG. 21C. Optionally or additionally,septa forming the top and bottom parts are moved.

Another un-illustrated three-part collimator is similar to collimator2128 of FIG. 22D, in which resolution is increased by tilting the septaof the central part.

The three layer configurations may be desirable in that they may providea more symmetrical high resolution pattern in the X and Y directions.

FIGS. 22A-22D illustrate embodiments in which the opening area of thecollimator cells is changed by shutter-like mechanisms. In FIGS. 22A and22B, collimator fragment 2200 is formed by X-Z plane septa 2202 and Y-Zplane septa 2204 that define collimator cells 2206. At the upper ends ofcells 2006 are pairs of cooperating triangular shutter leaves 2208 and2210. Shutter leaves 2208 may be fixed in place while leaves 2210 areslidable in the X direction, for example, by an actuator rod 2212. Inthe open position illustrated in FIG. 22A leaves 2210 substantiallyoverlie leaves 2208. In the closed position illustrated in FIG. 22Bleaves 2210 have been shifted to partially close the tops of cells 2006.

FIG. 22C illustrates an embodiment in which the tops of the collimatorcells 2206 are closed by rotating flaps or leaves 2214 on a mechanismshown schematically as an actuator rod 2216.

FIG. 22D illustrates an embodiment in which the tops of the collimatorcells 2206 are closed by an iris-like shutter 2218. This embodiment maybe advantageous in that it allows achievement of very small areaopenings for the collimator cells. Actuation of shutter 2218 may be by aconvention rotating mechanism, as in a photographic camera, or in anyother suitable and desired way.

The dimensions of the collimators illustrated in FIGS. 18A-22D may bevaried over ranges, for example, as indicated in the non-limitingexamples given below.

1. The pitch of the collimator septa may be in the range of about 1 mmto about 3 mm Larger or smaller pitch values are also possible.

2. Typical septa thickness may be in the range of about 0.2 mm to about0.3 mm Again, larger or smaller thicknesses are also possible.

3. The height of the septa (in the Z-direction) may be in the range ofabout 13 mm to about 25 mm, or larger or smaller values.

4. The length of the septa (in the X and Y directions) may range fromabout 8 mm to about 20 mm or larger or smaller values.

Exemplary Relationships Between Collimator and Pixilated DetectorConfigurations:

The following discussion describes some embodiments as non-limitingexamples of collimator configurations in relation to pixilateddetectors, using the above collimator designs or using othercollimators, such as machined slabs.

In a first embodiment, the collimator septa are aligned with thepixel-pitch of the detector, i.e., the septa are positioned at theborders of each pixel. In such a configuration, the collimator cells arealigned with the detector pixels with one pixel per collimator cell.

In another embodiment, the septal pitch in one direction, (e.g., the Xdirection as defined in connection with FIGS. 17-18A-18C), or the otherdirection (or both) is greater than the pixel pitch, for example, in aninteger ratio, of 1:2 or 1:3, or 1:4, or 1:5, or greater. In such anarrangement, if the septal pitch matches the pixel pitch in onedirection, and is two times the pixel pitch in the other direction,there will be two pixels within each collimator cell. If the septalpitch is twice the pixel pitch in both directions, there will be fourpixels in each collimator cell. This configuration may allow thegeneration of multiple different views within the same collimator cell.In other embodiments the septa are not aligned with some or all pixelboundaries.

In a further embodiment, all the septa in one or both the X-Z and Y-Zplanes (also as previously defined may be oriented so that they are notparallel to each other. This configuration forms a collimation structurein which multiple pixels have different views, passing through the samecell. In one example, a multiple pinhole structure is provided. In oneexample, multiple apertures are provided. In another example, themultiple apertures are arranged to form coded aperture collimationstructure, for example of a type known in the art.

It should be noted that coded aperture techniques are known to thoseskilled in the art for use in gamma ray imaging, and will not bedescribed here in the interest of brevity.

In another embodiment, the septa pitch is smaller than the pixel pitch,for example, according to an integer ratio, of 1:2, or 1:3, or 1:4, or1:5 or greater. This may be achieved by positioning one or moreadditional septa at the middle of a pixel to provide, multiplecollimator cells within each pixel. With this configuration it ispossible to obtain a particular viewing angle (collection angle) withshorter collimator septa. For example, if for a certain desiredcollection angle one would use a pixel pitch of 2 5 mm and a singlecollimator cell per pixel with septa length of about 20 mm, similarperformance can be obtained by providing two collimator cells per pixelwith a septa length of 10 mm Shorter septa may be advantageous sincethey may permit having a smaller detector head with bettermaneuverability.

In another embodiment, the collimator cell pitch is different in the Xand Y directions, and also different than the pixel pitch. For example,the collimator septa may be pitched at 2 mm in one direction and at 3 mmin the orthogonal direction. In general, the septa spacing may be largeror smaller than the pixel spacing and also different in the X and Ydirections/pitch. For example, N collimator septa may be evenly spreadover K pixels in the X direction, and M collimator septa may be evenlyspread over L pixels in the Y direction.

In a further exemplary embodiment of the invention the length of thecollimator septa is different in the X and Y directions. For example, inthe case of pixel pitch of about 2.5 mm, the X-Z plane septa can beabout 18 mm long and the Y-Z plane septa can be about 24 mm long. Withthis exemplary configuration it is possible for a pixilated detectorhaving square pixels to provide different view angles and collectionangles in each direction and to allow sensitivity and resolution ofreconstruction to be optimized where the camera scanning is veryasymmetric in its nature.

For example: a camera scan and reconstruction may form many X-Z “slices”along the Y axis which is parallel to patient body main axis and the Yresolution of the detector is very influential on the reconstructionresolution in the Y axis. This is different than the X-Z resolution, asthe depth dimension is obtained by different views and rotations withinthe X-Z plane, where the X resolution of detector is just one factor andthe distance, rotations, translations and reconstruction algorithmdetermine the resulting resolution. In these cases it may well be thatan improved result can be obtained with a collection angle which isdifferent in X axis than in Y axis. This is optionally achieved withthis unique approach of different septa length in the X-Z and Y-Zplanes.

Example

FIGS. 23A-23C illustrate qualitatively the result of a simulation studyperformed on an exemplary collimated detector as described herein formedof 0.2 mm tungsten septa. In FIG. 23A, the horizontal axis represents±θ, the angle between an approach path 2302 from an emission event 2303off the centerline 2304 of a collimator cell 2306 (see FIG. 23B), twocurves are shown: curve 2308 for a typical SPECT isotope, and 2310 for atypical PET isotope. As may be seen, SPECT performance is verydirectional, but for small values of θ, PET performance is comparable.

However, for PET isotopes, off-axis detection is reduced, but not by somuch as to prevent use of collimated detectors according to someembodiments hereof for selectable or simultaneous SPECT and PET imaging.

FIG. 23C provides an understanding of the saw-tooth shape of the offaxis detection probability for PET imaging. Here, it may be seen thatthe path 2310 for an emission event at some off-axis position 2308 mustpass through two septa 2312 and 2314 to impinge on the center of adetector pixel 2316, and suffers additional attenuation.

Moreover, having the reconstruction algorithm compensate for theoff-axis attenuation (e.g., with angle dependent sensitivity weighting,e.g., calculated and/or measured during calibration) can potentiallyimprove PET performance significantly.

As used herein the term “about” refers to ±10%.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

It is the intent of the applicant(s) that all publications, patents andpatent applications referred to in this specification are to beincorporated in their entirety by reference into the specification, asif each individual publication, patent or patent application wasspecifically and individually noted when referenced that it is to beincorporated herein by reference. In addition, citation oridentification of any reference in this application shall not beconstrued as an admission that such reference is available as prior artto the present invention. To the extent that section headings are used,they should not be construed as necessarily limiting. In addition, anypriority document(s) of this application is/are hereby incorporatedherein by reference in its/their entirety.

What is claimed is:
 1. A method of nuclear medicine imaging comprising:providing a nuclear medicine tomography system comprising: a support fora subject of a tomography procedure; a detector carrier; a plurality ofdetector heads mounted on said detector carrier, a first subgroup ofsaid plurality of detector heads each configured to be linearlyextended, from a first, most retracted position of said respectivedetector head relative to the subject, to a second respective position,said second position closer to the subject than said first position,wherein a second subgroup of said first subgroup of detector heads eachincludes a sensor configured to detect a proximity of a respective saiddetector head to the subject and a proximity of a respective saiddetector head relative to another said detector head; and a controllerconfigured to receive a signal from each said sensor and configured tocalculate a desired proximity of each said detector head of said secondsubgroup with the subject and with another said detector head; linearlyextending said first subgroup of detector heads from said respectivefirst positions to said respective second positions; wherein saidcontroller receives the signal from each said sensor of said secondsubgroup of detector heads and calculates an expected proximity of eachsaid detector head of said second subgroup with the subject, atrespective third positions, each said third position closer to thesubject than said respective second position; automatically linearlyextending said second subgroup of detector heads from said secondrespective positions to said respective third positions, based on saidcalculated expected proximity of each said detector head of said secondsubgroup with the subject at said respective third position; and basedon said calculation, perform one or more of: stopping said linearextension of at least one of said detector heads of said second subgroupprior to a collision between said at least one detector head of saidsecond subgroup and the subject or prior to a collision or interferencebetween said at least two said detector heads of said second subgroup;and slowing said linear extension of at least one of said detector headsof said second subgroup prior to a collision between said at least onedetector head of said second subgroup and the subject or prior to acollision or interference between said at least two detector heads ofsaid second subgroup.
 2. A method according to claim 1, wherein saidcontroller is configured to calculate and an expected proximity of eachsaid detector head of said second subgroup relative to another saiddetector head of said second subgroup; wherein said automaticallylinearly extending is additionally based on said calculated expectedproximity of each said detector head of said second subgroup relative toanother said detector head of said second subgroup.
 3. A methodaccording to claim 1, wherein said sensors are selected from the groupconsisting of contact sensors, pressure sensors, acoustic sensors, IRsensors, ultrasonic sensors, optical sensors, and distance detectors. 4.A method according to claim 1, wherein said slowing said linearextension includes slowing movement of said at least one detector headto a velocity below that which can cause injury to the subject.
 5. Amethod according to claim 1, wherein said controller receives an alertwhen a distance between a said detector head and the subject is below apreselected threshold or when a distance between at least two saiddetector heads is below a preselected threshold.
 6. A method accordingto claim 1, wherein said stopping includes stopping said linearextension of said at least one of said detector heads when a proximityof said at least one of said detector heads to the subject is less than20 cm.
 7. A method according to claim 1, wherein said stopping includesstopping said linear extension of said at least one of said detectorheads when a proximity of said at least one of said detector heads tothe subject is less than 10 cm.
 8. A method according to claim 1,wherein said slowing includes slowing said linear extension of said atleast one detector head while allowing contact between said at least onedetector head and the subject, a contact force between said at least onedetector head and the subject limited to less than 1000 g.
 9. A methodaccording to claim 1, wherein said slowing includes slowing said linearextension of said at least one detector head while allowing contactbetween said at least one detector head and the subject, a contact areabetween said at least one detector head and the subject limited to atleast 1-10 cm squared.
 10. A method according to claim 1, wherein saidslowing includes slowing said linear extension of said at least onedetector head while allowing contact between said at least one detectorhead and the subject, a contact between said at least one detector headand the subject limited to portions of said at least one detector headwithout sharp edges.
 11. A method according to claim 1, wherein saidinterference between at least two said detector heads includes at leastone of mechanical interference and operational interference.
 12. Amethod according to claim 1, wherein said controller allows saidlinearly extending to continue until a plurality of said detector headsis brought into proximity with the subject such that said detector headscover a solid angle around a region of interest (ROI), said solid angleselected from: more than 0.03 steradian, more than 0.05 steradian, morethan 0.07 steradian, more than 0.1 steradian, more than 0.15 steradian,more than 0.2 steradian, more than 0.5 steradian, more than 1 steradian,more than 1.5 steradian, more than 2 steradian, and more than 4steradian.
 13. A method according to claim 1, wherein said controller isconfigured to allow said linearly extending such that at least a portionof said plurality of detector heads detect a body contouring of thesubject without contacting the subject.
 14. A method according to claim1, wherein said linearly extending is actuated by operation of a motorand wherein said slowing said linear extension includes one of:activating a brake configured to slow said linear extension anddisengaging said motor from said linearly extending.
 15. A methodaccording to claim 1, further including, before said linearly extendingsaid first subgroup, selecting a first bore geometry for said tomographysystem.
 16. A method according to claim 15, wherein said selecting thefirst bore geometry is based on a region of interest of the subject. 17.A method according to claim 15, further including, after said linearlyextending said first subgroup, selecting a second bore geometry, saidsecond bore geometry smaller than said first bore geometry, whereat eachsaid detector head of said second subgroup is at a respective thirdposition, each said third position closer to the subject than saidrespective second position.
 18. A method according to claim 17, whereinsaid selecting the second bore geometry is based on a region of interestof the subject.
 19. A method according to claim 18, wherein saidautomatically linearly extending is based on said selected second boregeometry.
 20. A method according to claim 1, wherein said subjectsupport has an axis and wherein at least one of said subject support andsaid detector carrier is movable axially relative to the other.
 21. Amethod according to claim 15, wherein said linearly extending said firstsubgroup is based on the selected first bore geometry.
 22. A methodaccording to claim 1, wherein said detector carrier is rotatable about aregion of interest (ROI), said method further including, after saidlinearly extending, rotating said detector carrier and acquiring imagesof the ROI.
 23. The method according to claim 22, wherein said system isconfigured to provide a dynamically variable-geometry bore by providingdegrees of freedom for detector head configurations relative to saiddetector carrier, wherein said degrees of freedom are at least one of:preselectable before said acquiring imaging; and one of continuouslyadjustable during said acquiring images and step-wise adjustable duringsaid acquiring images.
 24. A method according to claim 1, wherein saiddetector carrier is tiltable relative to said subject support.